Catacomba di San Nicomede is a dubiously identified set of catacombs thought to be under the Ministry of Transport building at Via Nomentana 2 in the Nomentano quarter, next to the church of Corpus Domini al Nomentano.
Beware of an old identification of these catacombs with the Ipogeo di Via dei Villini, at Via dei Villini 32 nearby. This is still active online, but is almost certainly wrong.
The revised Roman Martyrology (2001) has this entry for 15 September:
The saint was a Roman martyr about whom nothing is known. No legend was written for him, although he occurs as a character in that of SS Nereus and Achilleus. There he is described as a priest flogged to death after refusing to sacrifice to the gods, but this legend is fictional. It was quoted in the old Roman Martyrology, but has been deleted.
The first possible mention of a church arises in the 5th century, when there existed a titulus Sancti Nicomedis. However, the main data comes from the 7th century pilgrimage itineraries. Three of these mention the saint's shrine, and the Topographia Einsiedlensis gives the useful detail that it was on the right side of the road when leaving the city.
Pope Adrian I (772-95) restored the church, which in the Liber Pontificalis is described as ecclesiam beati Nicomedis, sitam foras portam Numentanam. This gives the further detail that the church was outside the present Porta Pia, so archaeologists have been looking for the catacombs here since the start of the 17th century. The results have been confusing.
Antonio Bosio wrote that he found catacombs in the expected location, but gave no details and nobody else noticed. Hence, it is uncertain what he actually discovered.
Unfortunately, serious archaeological interest only began when the area was about to be built over. Giovanni Battista De Rossi conducted investigations which he finished in 1864 and wrote up the following year. The locality which he examined was then the rural Villa Patrizi, soon to be redeveloped for what is now the Via dei Villini. He found a hypogeum and a small set of catacombs, and also the foundations of an apsidal building which he identified as the church of Pope Benedict. Epigraphs that he found are of the late 2nd and 3rd centuries.
De Rossi transcribed the most important one, which probably came from a ground-level mausoleum:
Monumentum Valeri Mercuri et Iulittes Iuliani et Quintilies Verecundies, libertis libertabusque posetrisque eorum at religionem pertinentes meam. Hoc amplius in circuitum circa monumentum lati longe per ped[es] binos quod pertinet at ipsum monument[um].
("The monument of Valerius Mercurius and Julitta Juliani and Quintilia Verecundia, for their freedmen and freedwomen and successors belonging to my religion. The further area around this monument for two feet in length and width pertains to this monument.")
This indicates that the little set of catacombs was a completely private family affair. Freed slaves counted as part of the family under ancient Roman patronage rules.
No overtly Christian artefacts or decorations were found in the excavation, and it is impossible to decide which religion is referred to in the epigraph above.
The building work that followed at the end of the 19th century destroyed the apsidal ruin, and also the hypogeum (apparently), but left the catacombs intact. They are under a convent building put up in 1900, the Convento Notre Dame des Oiseaux.
De Rossi's hypothesis that the little set of catacombs were those of San Nicomede was generally accepted, until the right side of the Via Nomentana immediately outside the Porta Pia was itself developed. A massive office building was finished in 1918, initially for the Italian State Railways and later functioning as part of the accommodation of the Ministry for Transport.
The digging of the foundations revealed a set of catacombs on two levels, allegedly with thirty or so passages. Very unfortunately the timing of the project did not allow for archaeological investigations, and this was before the Vatican was given privileged ownership of the city's catacombs in 1929. The upper level was reported as destroyed for the foundations, and the lower level sealed off.
The Via Nomentana catacombs seem to have been destroyed without any plan or survey being made.
The little set of catacombs on the Via dei Villini illustrate the semantic distinction between ipogeo and catacomba. In the literature the former is often used as referring to private burial places underground, and the latter to those belonging to the Church. However, all catacombs began as private enterprises and the distinction is better served by focusing on whether the complex has a set of passages with loculi. Under this definition, the Via dei Villini site amounts to a very small set of catacombs.
De Rossi describes them as having their original staircase entrance, leading to a wide passage lined with masonry which ends in a cistern. Along the right wall of the passage is a ceramic water pipe, made from amphorae joined up with their bottoms broken off and which turns down a conduit to the right which has a small cross-section. This set of excavations De Rossi thought, with justification, to have been part of the underground services of an ancient Roman villa pre-dating the catacombs.
When the underground complex was taken over for funerary use, perhaps in the late 3rd century, three cubicula were excavated off the main passage. Two are to the left, and one on the right which used to be richly decorated with marble work. This led De Rossi to surmise that the shrine of St Nicomedes was here.
Further down the main passage, the pipe conduit on the right had some loculi excavated into its sides, and a short side gallery with further loculi was also excavated.
The Via dei Villini site would be worth a visit if the locality were an ordinary archaeological site with a modest entry fee, but unfortunately it is part of the Vatican discretionary scheme for catacombs not open to the public. This allows visits to accessible closed catacombs by groups of not more than fifteen, "for a real and exclusive cultural purpose". The minimum fee in 2015 is 220 euros, increasing after 75 minutes. See the PDF file Rules Regarding Visits to the Catacombs Closed to the Public.
While interesting, these catacombs are certainly not worth that sort of money and effort to see.
Almost all the dedicated material online refers to the Ipogeo di Via dei Villini.