Santa Maria in Via Lata is a 17th century minor basilica and titular church of ancient foundation (formerly parochial and collegiate, now conventual) at Via del Corso 306 in the rione Pigna. This is in the north-east corner of the city block occupied by the Palazzo Doria Pamphilj. Pictures of the church at Wikimedia Commons are here. There is an English Wikipedia page here.
The dedication is to the Blessed Virgin Mary under her title of Our Lady, Advocate (Madonna Advocata).
The name Via Lata does not refer to the little street to the north of the church, which is actually named after the church and not the other way round. Rather, it is the ancient name of the present Corso.
It is easy to confuse this church with Santa Maria in Via nearby.
The Via Lata, the present Corso, was the urban section of the Via Flaminia to Rimini and was one of the most important streets in ancient Rome. The ancient ground surface is just over five metres below the present street, and the church stands on remains of a massive structure part of which was re-used as a church institution at the end of the 7th century.
What we have here is part of a multi-storey structure of the Hadrianic period, 1st century AD. It is not clear whether it was a typical insula with commercial premises on the ground floor and apartments above, or whether it had a public function. The former seems to be the choice of the modern scholarly consensus. What is preserved under the church is part of a pillared gallery twenty-eight metres wide, with four rows of travertine piers parallel to the street which divide this into a central zone fourteen metres wide, and two side aisles of seven metres each. Krautheimer in 1971 proposed that the edifice occupied the entire city block between the present Via Lata and the Vicolo Doria (his hypothetical plan is here).
The original vault was about ten metres high, indicating that the building might originally have been a horrea or warehouse. In the 3rd century two storeys were made of this by inserting a vault at a height of 5.5 metres, and at least under the church the area was divided into small rooms by inserting brick walls in between the piers. There is also evidence of a wooden mezzanine floor further sub-dividing the five-metre high space. This looks like accommodation for small shops and businesses.
At this point on the Corso used to stand the Arcus Novus, a triumphal arch erected by the emperor Diocletian in 303-304 to celebrate the twentieth anniversary of his accession. It was destroyed in 1491.
The tradition attached to the site claims that St Paul spent two years here when he was under house arrest, waiting for his trial before the emperor (see the end of the Acts of the Apostles). This conflicts with a similar tradition concerning San Paolo alla Regola. An embellishments to the story is that St Luke the Evangelist wrote his Gospel and the Acts here as St Paul's secretary. Further, that he painted the portrait of Our Lady which is venerated in the church (this is demonstrably false).
A separate tradition claims that St Peter, and Peter's disciple Martial, first bishop of Limoges, were incarcerated here with St Paul before being lodged in the Mamertine Prison. St Martial was actually of the mid 3rd century. A well is pointed out as one that miraculously sprung up in order that St Peter could baptize visitors.
The origins of the church are actually as a diaconia, which was an early church equivalent of a modern social services centre. Here, needy people could be helped and necessities such as food given out. The person in charge of one of these was a diaconus, deriving from the Greek for a servant, and was not the same thing as a later deacon as found in the liturgy. However, most of these diaconia were to evolve into titular churches (including this one), even though they were not originally tituli.
When the diaconia was established, one of the rooms was converted into a little chapel by adding an apse. This was the first Christian place of worship here, and can be visited underground. Five other excavated rooms were added to the complex, also visitable.
Dating is a problem. A foundation date in the 4th century has been quoted, but the evidence is lacking. The frescoes found in excavations date mostly from the late 7th to the 11th centuries, but are palimpsests and certain fragments have been claimed as dating to the end of the 6th century.
The earliest documentary mention is in 806, but a 17th century description asserts that the actual foundation was in the reign of Pope Sergius I (687-701), who enshrined a martyr called St Agapitus here. Alternatively, that one Theophylact, husband of Theodora who was a sister of Alberic a Roman senator, founded it in 706 (these names are more familiar in early 10th century Roman history, the Pornocracy -see Theophylact I ).
If the above fresco analysis is accepted, then we seem to have a possible late 6th century foundation for the diaconia. However, the dating is debateable and the consensus seems to be that the Sergius foundation tradition is reliable. The church is nowadays not regarded as being of palaeochristian (that is, pre-8th century) foundation.
The original dedication seems to have been to SS Paul and Luke.
It is thought that the complex became a Byzantine-rite monastery in the mid 8th century, staffed by refugee monks from the iconoclastic persecutions of the Byzantine Empire. There are hints of Eastern influence in the underground frescoes, notably a depiction of the Seven Sleepers of Ephesus.
The palimpsest nature of these frescoes indicate that the were at least two major restorations before the church was rebuilt in the 11th century, involving overpaintings. These might correlate to recorded benefactions to the church by popes, starting with Leo III in 806 and then by Gregory IV (827-44) continued by Sergius II (844-7). Again, we have donations by Benedict III (855-8), continued by Nicholas III (858-67).
Flooding became a problem as the street level rose owing to general ruination of the city's fabric, and so the church was rebuilt at a higher level in 1049 in the reign of Leo IX. Other churches were treated in the same way for the same reason -see San Clemente and San Crisogono. However what was unusual here was that part of the old monastery including the original little chapel was preserved as a crypt, while other parts were walled off and filled in. The accessible bit comprised two rooms, the old oratory (with its apse walled off) and the room to the south. These are known nowadays as Rooms I and II.
The church's orientation was in the opposite direction to what it is now. The apse was next to the Arcus Novus on the Corso, while the nave was shorter than at present.
The Byzantine-rite monks would have left in the 10th century. After the rebuilding, the church was parochial and was administered by a college of secular priests.
The dedication to Our Lady arose at the start of the 15th century, owing to popular veneration of the icon now over the high altar. This apparently came about after an apparition of her in 1408.
Just to the west of the church, on what is now the east end of the Piazza del Collegio Romano, was a very important Benedictine monastery in the Middle Ages. This was known as San Ciriaco de Camilliano, or Santi Ciriaco e Niccolò because it incorporated an older church called San Niccolò de Pinea. It was founded for monks in the 10th century, but by the 13th century was inhabited by nuns.
It claimed the relics of St Cyriac, formerly enshrined at San Ciriaco a Via Ostiense, together with companion martyrs Largus and Smaragdus. Further, it was a Lenten station church and also claimed descent from the lost ancient titulus of San Ciriaco, which was actually near the present Termini train station.
However, in the 15th century the nuns had become grossly degenerate. As a result, the nunnery was suppressed as a disgrace and the church closed by a papal decree of Eugene IV in 1435. The Lenten station, and the relics of St Quiriacus, were transferred to Santa Maria in Via Lata.
It is unusual for a church of such high status to have been suppressed in this way. The Palazzo Doria Pamphilj was to take over much of the site, but that lay in the future. Nuns indulging in recreational sex and having babies were not unknown at the time (witness the Venetians, who had plenty of them), and the real reason might have been the practice of black magic by the nuns here.
The name is also given as Quiriaco.
Second rebuilding 
The church was rebuilt from 1491, on the orders of Pope Innocent VIII, after it had fallen into dereliction. The ancient Arcus Novus was demolished as part of the project, as the church's orientation was reversed so that the entrance façade replaced the sanctuary apse. Also, apparently the church was extended to the west over the site of the old monastic church of San Ciriaco, which must have been right up against the previous edifice.
It is on record that this church was a prestigious building, with arcades supported by ancient columns of cipollino marble. SS Quiriacus, Largus and Smaragdus were re-enshrined here.
The cardinal of the church in charge of the rebuilding was Rodrigo de Borja, later Pope Alexander VI. The work was completed in 1506, but a campanile was added in 1580 to a design by Martino Longhi il Vecchio.
In 1594 the crypt was restored by Agostino Gasoli, and the present Rooms V and VI cleared out.
Pope Alexander VII ordered another rebuilding only just over a hundred and thirty years later, in 1639. The supervisor was Atanasio Ridolfi, a canon of the church, and architect was Cosimo Fanzago, who finished work on the structure in 1647 and on the interior in 1650. (He is much more well-known for his work in Naples.) The façade was a separate undertaking, by Pietro da Cortona , from 1658 to 1662.
Part of this rebuilding project involved the renovation of the crypt for devotional purposes, since the pope took the legend seriously. He re-consecrated the crypt chapel in 1661, with a new altar bearing a marble bas-relief by Cosimo Fancelli. The loggia of the façade was provided with two sets of side stairs leading down into the crypt, and for the purpose the present Room III was cleared out so as to receive the right hand stairs.
The church has had a quiet existence since the 17th century, and since it was not conventual was not disturbed by the troubles of the 19th century.
In 1863 there was a restoration of the interior supervised by Salvatore Bianchi.
In 1905, there were archaeological investigations in the crypt which involved the clearing of Room VI. A further room, Room VII, was noted but not cleared. Several of the ancient frescoes were discovered then.
In 1909 the parish was suppressed because its continuation could not be justified. The parish territory here was mostly occupied by palazzi and church institutions, and had almost no ordinary parishioners.
In 1960 certain of the frescoes were removed because of deterioration, and found to be palimpsests with older ones underneath. This is why some frescoes are still here, and others at the Crypta Balbi museum.
The church is now conventual, since it is cared for by a community of religious sisters called Daughters of the Church (Figlie della Chiesa) who have their headquarters at Santa Maria Mater Ecclesiae a Viale Vaticano. They founded a convent here in 1969 dedicated to Our Lady, Mater Unitatis, and run a Eucharistic Ecumenical Centre (Centro Eucaristico Ecumenico).
Also, since 1974 the complex is the location of the Marian Cultural Centre (Centro Cultura Mariana), an initiative by P. Ermanno-Maria Toniolo of the Servite friars.
Traditionally the cardinal deaconry began in 250, but this is unhistorical. The first documented cardinal was actually a pseudocardinal, later Antipope Theodoric in 1100.
The current titular deacon of the church is Edward Idris Cardinal Cassidy, titular archbishop of Amanzia, who was appointed deacon in 1991 and who has held the title as a priest pro hac vice since 2002.
Layout and fabric
The layout is classically basilical, having a nave of seven bays with side aisles, and a semi-circular apse. A small convent block is attached to the left hand side.
There is a chapel at the end of each side aisle, but no structural side chapels. Instead, there are four side-altars in niches within the fabric of the side walls.
The fabric is in brick, with architectural details in limestone. The nave and aisle roofs are pitched and tiled, although part of the left hand aisle has part of the convent block over it. The apse is incorporated into a wing of the Palazzo Doria Pamphilj.
The entrance block is an architecturally separate add-on, and is substantially higher than the main body of the church.
The fabric is in brick rendered orange where it is exposed, with architectural details in white and carved bits in travertine limestone. This stone is used for facing the entrance block.
Via Lata frontage
On the Via Lata, the right hand side wall is given an entablature with a projecting cornice and a Corinthian pilaster at each end. The wall has seven sections corresponding to the nave bays, three of which are blank. The other four have lunette windows within molded archivolts flanked by Doric pilasters supporting the entablature. These windows have an odd shape, because each has a semi-circular protrusion on the sill. The reason for this design feature lies inside.
This side wall has two coats-of-arms in shallow relief, one of Pope Innocent VIII and the other of Cardinal Rodrigo de Borja before he became pope. These are from before the 17th century rebuilding.
The tower campanile by Martino Longhi il Vecchio is the oldest part of the above-ground church structure, and is attached to the far end of the left hand side wall of the second storey of the entrance block.
The first storey is part of the building next to the left hand aisle of the church, and the second storey is a tall cuboid with an empty round-headed niche on each face flanked by a pair of blind pilasters at the corners supporting an entablature with a strongly projecting cornice. The third storey, the bell-chamber which towers above the entrance block, is more interesting. Each face has a tall round-headed soundhole, flanked by a pair of Ionic pilasters at the corners -except that the surfaces of the pilasters are actually strap curlicues with incurved volutes below the capitals. Each face has a segmental pediment on top, and finally there is an ogee cupola on a little circular drum.
There are three bells, two of which bear dates: 1615 and 1465.
The entrance block, with its monumental façade and internal loggia is by Pietro da Cortona
and was finished in 1662. The work was of great interest to Pope Alexander VII whose family, the Chigi, hoped to develop a palace and private church complex in the area along the lines of that of the Pamphilj in Piazza Navona which resulted in Sant'Agnese in Agone.
There are two storeys, of the same width. The central portion of each is a colonnaded portico, flanked by narrow side zones set back slightly. The structure is entirely in travertine limestone, and has a tiled roof which is pitched and hipped.
The first storey has the internal loggia taking up the entire height. The void is fronted by four monumental Corinthian columns with superbly carved capitals, and is bounded by triplet pilasters in the same style. The recessed side zones are beyond these, and have a further pair of pilasters at the corners. Two large vertical rectangular windows with molded Baroque frames topped by entablature fragments are fitted snugly in between the triplet and the outer pilasters.
Columns and pilasters support an entablature which runs across the entire width of the façade, separating the storeys. The frieze of this bears a dedicatory inscription: Deiparae Virgini Semper Immaculatae MDCLXII ("To the God-bearing virgin, always immaculate, 1662"). The cornice projects more than is usual, and the central section is supported on a pair of posts above the triplet pilasters.
The curiously designed second story has another loggia, the void of which is defined by an entablature supported by columns. This entablature has a central arched section flanked by a pair of trabeations (horizontal bits), the design being called a serliana. On the arched section is a scroll bearing the name of Pope Alexander VII.
The entablature of the serliana is supported by four columns matching those below. This middle section of the storey is bounded by -not tripletted pilasters, but by a pair of columns in front of double pilasters. The cornice of the entablature is part of the crowning triangular pediment, into which the central arch intrudes and which only spans the serliana. A pair of pilasters occupy the corners of the recessed side zones of this storey also, and in between these and the triplets is a pair of empty round-headed niches.
There are four flaming urn finials on top of the façade, and a crowning central metal cross.
The architectural device of a colonnaded upper storey loggia was associated with public audiences given by Roman emperors, and was also utilized by them for their viewing-boxes at the Circus Maximus. This might have been, in part, the appeal of the design to Pope Alexander VII, whose large-scale redevelopment of the area focused on improving the Corso as the main axis of the city and the site of horse races during Carnival. In other words, the upper storey of this façade was possibly intended as a private viewing-box. It proved useful to the church's clergy in subsequent centuries as an al-fresco space, since the church has no garden attached.
Once through the wrought iron gates guarding the entrance loggia, you find yourself in a space the shape of a rectangle with rounded corners. The 17th century entrances to the underground zone are to the right and left. The actual church entrance is flanked by a further four Corinthian columns, and over the door is a circular tondo containing a relief of Our Lady. This is by Cosimo Fancelli.
There are two tablets with epigraphs here, commemorating the 17th century rebuilding project. One details the traditions associated with the underground are, together with how the pope had it restored, and reads:
Alexandro VII Pont. Maximo, locus antiqua veneratione sacer et nobilis in quo S. Paulum apostolum diu moratum non semel, una cum ipso ecclesiae capite S. Petro de rebus Christianae fidei deliberasse, ubi S. Lucam evangelistam et scripsisse et deiparae Virginis imagines depinxisse, iam inde a primis temporibus traditum, congestu terrae olim depressus atq. inaccessus, facili scalarum descensu, immissoque fenestris lumine pervivus factus, perpurgatus exornatusque pio fidelium cultui restitutus est, anno sal. MDCLXI.
The other summarizes the later history of the church and the pope's rebuilding of it, and reads:
Ecclesia Sanctae Mariae in Via Lata, olim basilicae nomine inter caeteras Urbis vetustate, et canonicorum collegio insignis, a Sergio PP I diaconiae titulo consecrata, sub altari corpore S. martyris Agapiti collocato, a Leone IX multis sanctorum ditata reliquiis, ab Eugenio IV monasterio et ecclesiis SS. Cyriaci et Nicolai ipsi unitis aucta et locupletata, ab Innocentio VIII novo aedificio in ampliorem formam redacta, atque ab aliis summis pontificibus donis spiritualibus cumulata, demum ab Alexandro VII magnifice instaurata est et ornata, anno sal. MDCLXII.
To the left is a monument to Atanasio Ridolfi, 1663 by Pietro da Cortona. He was the canon of the church who initiated the 17th century rebuilding and apparently paid for much of it.
It is worth looking up into the barrel-vaulted ceiling of the loggia. It is coffered in rosetted octagons and squares, and over the crypt staircases is a pair of conchs or semi-domes with radial coffering and festoons on the archivolts. The colour scheme is white and light grey, and is a complete and possibly deliberate contrast to the riot of colour in the church itself.
The decoration of the interior is spectacular in its richness. It was supervised by Cosimo Fanzago between 1647 and 1652, when he was in exile at Rome. He had got into serious trouble in Naples, his home city, and had to flee for his life. Later he was able to return there and resume his career, as perhaps the greatest Neapolitan Baroque architect.
The nave has seven bays with side aisles, and the arcades are supported by a total of twelve Corinthian columns in red and white Sicilian jasper with gilded capitals. The archivolts of the arcades do not spring from imposts, but from the column capitals. Above them is a floating entablature, unsupported by pilasters and without an architrave, and this runs round the entire church. The nave side walls above this in turn have windows, the central pair being round.
On the wall between the arcades and entablature, on the arch intradoses and in between the windows is much gilded stucco decoration.
The flat wooden ceiling is richly coffered, gilded and painted with large cassettoni in different shapes. The central fresco depicting the Coronation of Our Lady is by Giacinto Brandi , 1650. There was a restoration of this ceiling in 1863, as an epigraph in the coffer nearest the entrance points out (as well as indicating that Pope Pius IX put up the funding).
The organ over the entrance has a richly carved and gilded case, and was a donation by the same Giovanni Battista D'Aste who paid for the high altar.
Unusually in a Roman church of the 17th century, the collegiate choir is in front of the sanctuary (it is usually to be found behind the high altar), and occupies the last two bays of the nave. The wooden choir stalls were executed by Francesco Speranza in 1628.
The Cosmatesque-style polychrome marble floor here is a survival from the 15th century church.
On the cornice of the entablatures above the entrance of the choir sits a pair of stucco angels holding torches. These have been attributed to Bernini in the past, and might be of his school.
The ceiling coffer over the choir contains a trompe-l'oeil fresco by Viviano Codazzi, which gives the impression of looking from below into an open pavilion on the plan of a chamfered rectangle, with statuary and the sky visible through the arches to the sides. An angel and a putto are disporting themselves. Note that the arches are serlianae, recalling the design of the second storey of the façade. The label Assumpta est Maria in coelum refers to the fresco in the conch of the sanctuary below, where Our Lady is waiting to ascend into her heavenly pavilion.
The altar has also been attributed to Bernini, but is now though to be by Santi Ghetti. It was erected in 1643 at the expense of Giovanni Battista D'Asti. The aedicule is coved (convex), fitting the curve of the apse wall, and has four Corinthian columns in alabaster. These support a segmental pediment containing a pair of putti holding a crown for the icon below, and on this pediment sit two allegorical figures in marble. Gentleness is on the left, and Charity on the right.
In a very elaborate Baroque frame is the venerated icon of Our Lady Vergine Advocata, which is said to have elicited many miracles. It is late 12th century, with an inscription saying Fons lucis stela maris (sic -"fountain of light, star of the sea") and unusually is signed: Petrus pictor. The style is called the Aghiosoritissa in the Byzantine tradition, but here Our Lady is gesturing to the left -instead of the right as would be expected (the wider scope is that she is standing at the right hand of Christ and supplicating him). This oddity is shared by the similar icons at Sant'Alessio all'Aventino and Santa Maria del Rosario a Monte Mario.
The relics of the 3rd century deacon and martyr St Agapitus are enshrined beneath the altar, allegedly having been brought here by Pope Sergius I at the start of the 8th century.
The conch of the apse has a fresco of the Assumption of Our Lady, originally by Andrea Camassei but was altered by one A. Traversari in the 18th century. Apparently the work by Camassei depicted The Eternal Father, in which case Traversari turned him into Our Lady. Further, if this is correct then the Codazzi trompe-l'oeil in the ceiling above must have been altered as well to fit the scheme.
The altar aedicule is flanked by two similar wall monuments, one for Giovanni Battista D'Aste the donor and the other for his wife, Clarice Margani. The bronze busts, partly gilded, are by different sculptors of the school of Bernini. His is by Giuliano Finelli, and hers by Andrea Bolgi who is more famous for his statues in St Peter's. These two works deserve to be better known.
Unlike the main nave, the aisles are cross-vaulted. The vaulting springs from wall pilasters opposite the nave columns and in the same style, red jasper with gilded capitals. The vaults are painted in what looks like 19th century style, featuring cornucopias and foliage sprays with scallops all in gold on green with red bordering containing putto's heads, the vault rib angles being highlighted in pale blue.
The walls of the bays of the aisles have alternate styles. Four of the bays have lunette windows into which intrude large circular tondi containing paintings and supported by pairs of gilded putti. This design feature accounts for the odd shape of the windows as seen from outside. The other three bays of each aisle have either altars (the first two) or pictures (the far one).
The near end of each aisle has a round-headed niche with another circular painting over it, larger than the ones on the side wall, and the far end has a chapel flanking the sanctuary.
The arrangement of the side wall tondi has doctrinal significance. The right hand tondi show events from the life of Christ, and the left hand tondi corresponding ones from the life of Our Lady. The doctrine of Our Lady as the Mediatrix of all graces is being hinted at.
Right hand aisle
The large circular painting at the near end of this aisle is The Baptism of Christ by Agostino Masucci.
The tondo in the first bay is painted by the same artist, and depicts The Annunciation of the Birth of Christ.
The second bay contains an altar dedicated to St Andrew the Apostle, and the altarpiece by Brandi depicts his martyrdom.
The tondo in the third bay is by Pietro da Pietri, and depicts The Nativity.
The tondo in the fifth bay is by da Pietri, and depicts The Presentation of Christ.
The sixth bay has a large elliptical painting of Our Lady of the Rosary by Giovanni Domenico Piastrini (1678-1714).
The tondo in the seventh bay is by Masucci, and depicts The Adoration of the Magi.
Monuments in this aisle include those to: Cardinal Maurizio di Savoja, 1637; Edward Dodwell, 1832, and Jean-Germain Drouai, 1788. The last was an artist of note, and his memorial is by Claude Michallon.
Chapel of the Crucifix
The chapel at the end of this aisle is dedicated to the Crucifixion, and has a 17th or 18th century wooden crucifix as an altarpiece. On the altar is a superb polychrome marble tabernacle in the form of a circular temple with a pedimented prothyrium and an ogee dome. The little columns are in alabaster.
The picture on the left hand wall here is of Our Lady of Sorrows, as you might expect, but instead of St John the Evangelist the right hand wall has one of St Cyrus. Together with his companion John, he is especially venerated by Copts in Egypt as a medical doctor who treated poor people for free.
The arrival of some of their relics at Rome lead to the foundation of the old suburban church of Santa Passera.
Left hand aisle
The large picture at the near end of the aisle is by Piastrini, and depicts The Risen Christ.
The tondo in the first bay is by Masucci, and depicts The Annunciation of the Birth of Our Lady to SS Joachim and Anne.
The tondo in the third bay is by da Pietri also, and depicts The Birth of Our Lady.
The tondo in the fifth bay is by Masucci, and depicts The Presentation of Our Lady at the Temple.
The sixth bay has no picture.
The tondo in the seventh bay is by Masucci again, and depicts The Marriage of Our Lady and St Joseph.
A memorial to the poet Antonio Tebaldeo (1453-1537) is at the end of the left aisle. It was designed in 1776. He was a friend of Raphael, who painted a portrait of him of which a copy is found here; the original is in the Vatican Pinacoteca.
Chapel of St Quiriacus
The chapel at the end of this aisle is the descendent of the demolished church of San Ciriaco, and contains the relics of St Quiriacus of Ostia. For his original shrine at Ostia, see Oratorio di San Quiriaco.
The altar has a pair of Ionic columns in verde antico, and an altarpiece depicting Our Lady with SS Catherine of Alexandria and Cyriac is by Giovanni Odazzi, 1716.
The sacristy is off the left hand aisle. It has a terracotta relief of The Rest on the Escape to Egypt by Fancelli.
The underground area of the church has always been accessible in part throughout the centuries and venerated as the house of St Paul. It was restored when the church was rebuilt in the 17th century, and an altar relief by Cosimo Fancelli provided, but serious archaeological investigation only began at the start of the 20th century.
The remains of a large ancient Roman structure, perhaps some 250 meters long, were tentatively mapped out, and the late 7th century chapel and diaconia (welfare centre) that were constructed within this were identified. There are murals from the 7th to the 11th centuries, some of which have been detached from the walls and taken to the Crypta Balba to preserve them. Photographs have replaced these.
The access is to the side of the entrance loggia. You have to pay an entrance charge; note that the custodian is not able to help with access to the church itself if it is closed. The staircase by which you descend is 17th century.
What you can visit is a set of six rooms. It must be made clear that this is only part of the original establishment, which certainly extended into the unexcavated area under the loggia and very likely to each side as well. A plan is here.
The chambers are given Roman numerals: Rooms I to VI, with I to III from north to south (the church's left to right) next to the foundations of the façade, and IV to VI in a corresponding row behind to the west (towards the altar of the church).
Room I is the traditional Prison of St Paul, and contains a small free-standing ancient Composite column which now has a marble cinerary urn with the Chi-rho monogram sitting on top. The column has a cross incised on it, and a spiral inscription that reads: Verbum Dei non est alligatum ("The word of God is not bound"). Marks of rust on the column indicate that it once had an iron chain wrapped around it, and the chain was actually found in the adjacent well in 2010.
The well is, by legend, the one that St Peter used for baptisms. It has a crudely carved cylindrical well-head with an octagonal marble rim, and still contains water.
The second room is the original diaconia chapel, with a walled-off apse to the east (on the right as you enter from Room I). By the stairs to the next room (Room III) is a pluteus or marble panel carved with a cross motif on one side, and a cross-flower (crucifer) on the other. A fragment of another pluteus is also in this room, this one pierced. These slabs were used on-end as screens for church sanctuaries in the Dark Ages.
To the left of the pluteus is an ancient square altar, and then a second free-standing column. There were extensive ancient frescoes in this room, now badly decayed but a photo shows one that featured three popes or bishops to the left of the altar which was late 8th century. Over the altar was a fresco of Christ Crucified.
The wall containing the doorway to Room V had a palimpsest fresco of three layers featuring different themes, one of them being the Multiplication of the Loaves and Fishes, 7th to 8th centuries.
Room III is where the right hand staircase from the entrance loggia ends up. It is now not very interesting, because the frescoes it once had have rotted away.
Room IV is the room behind the room with the well. It contained an important fresco cycle of the Legend of the Seven Sleepers of Ephesus, 7th century. This was overpainted with a fresco of the martyrdom of St Erasmus in the 8th century.
The west wall in here has an arched niche flanked by two late 8th century fresco figures who represent SS John and Paul. To the right of these there is a little structure consisting of two marble pillars with a lintel, possibly part of a former altar. One pillar has Cosmatesque decoration.
To the south of this room is a known Room VIII, which has not been explored.
This room was converted into the main underground chapel in the 17th century, and has a Baroque altar with a marble reflief by Cosimo Fancelli showing SS Peter and Paul reminiscing while St Luke listens in order to write his gospel, with St Martial in the background.
The last room in the second row of rooms, Room VI, was converted into a sacristy in the 17th century by re-facing the walls. The niche in the west wall now contains a fresco of that date depicting Our Lady between SS Peter and Paul; this apparently replaced an ancient fresco of the same theme. However, the north wall has been left untouched and consists of massive ancient limestone blocks.
This room now contains an ancient free-standing altar decorated with Cosmatesque mosaic work in mediaeval times.
The church is open (according to its website, July 2018):
Daily, 17:00 to 22:15.
However, in "Summer" (July and August?) it opens at 18:00.
The underground area has separate opening arrangements, and access is at the right hand side of the entrance portico:
Tuesday to Sunday, summer 16:00 to 19:00. Winter 15:00 to 18:00.
Also Saturday morning, 10:00 to 13:00.
There is a small charge for entry. The above details supersede previously advertised arrangements.
Some frescoes from the early diaconia and church are on separate display at Rome's Crypta Balbi museum, necessary because they overlaid those here now.
There is a daily programme of evening worship, as follows (church website, June 2018):
17:00 Exposition and Adoration of the Blessed Sacrament.
21:30 Further Adoration of the Blessed Sacrament.
Please do not wander about the church during these. Those who are not Catholic are reminded never to walk between the Blessed Sacrament and the congregation when the former is exposed for veneration, because this causes serious offence.
"casey-johnson" page on his graphics portfolio (contains good plans and elevations)