Catacomba di Domitilla is a catacomb named after one of the two martyrs named Domitilla, most likely Flavia Domitilla whose relics were moved to Santi Nereo e Achilleo. Pictures of the catacomb at Wikimedia Commons. 
The catacomb was originally a cluster of underground burial chambers, which were joined into a single network. Some of the first chambers belonged to pagan families who converted. There are more than 12 km of galleries here.
Around 390, a large underground basilica was built in honour of the soldier martyrs Sts Nereus and Achilleus.
In 1874, the basilica was rediscovered and subsequently reconstructed.
The catacomb is served by the German Brothers of Mercy. Those that assist at Mass are usually invited to breakfast in their lovely garden, or indoors in the winter.
Basilica of Sts Nereus and AchilleusEdit
The columns dividing the nave into three aisles has capitals that were reused from older buildings.
The small columns were originally part of a canopy above the high altar. On one of them, a relief of the martyrdom of St Achilleus has been carved.
Sts Nereus and Achilleus were originally interred in a private chamber below the altar, but their relics were later brough to Santi Nereo e Achilleo, where they are buried beneath the high altar.
The catacombs became unsafe in the Middle Ages, and the basilica was abandoned in the 9th century. Giovanni Battista de Rossi discovered it in 1874, and started excavations. It has been reconstructed, especially in the upper parts and the ceiling.
There are remains of an early Medieval schola cantorum, or choir enclosure.
The sarcophagi and insciptions here have been found in the area.
Cubiculum of St PetronillaEdit
Many wall-paintings have been preserved in the catacombs, and one that is usually shown during the tours depicts the Roman lady Veneranda being introduced into Paradise by her patron, St Petronilla. This 4th century painting has an historical interest, since it documents the veneration of saints among the lay people, and in particular it documents that the concept of patron saints is an ancient tradition. The identity of St Petronilla is uncertain, but she was known as the succour of the Frankish nation, and during the pontificate of Paul I (757-767) her relics were moved to the Chapel of the Frankish Kings in San Pietro in Vaticano, which was demolished with the old basilica.
Burial chambers of the FlaviiEdit
These chambers were originally used for pagan burials. During the 3rd century, Christians were also buried here. It's name is probably incorrect - de Rossi believed that members of the Flavii family, to which Domitilla belonged, where buried here, but there is no evidence of this. Cubiculum of the Great Apostles
Another painting often shown is of Christ seated with the Apostles, also from the 4th century.
This catacomb also has a very fine mosaic, probably the finest in any catacomb. It shows Christ between Sts Peter and Paul. It was mentioned in 1742 by Marangoni, and was recently rediscovered during a landslide, and is normally not shown during tours. If you wish to see it, try to make an appointment. This is always easier off-season, as there is less pressure on the guides.
The Ardeatine CavesEdit
While not related to the catacomb, this memorial that lies nearby should be mentioned, as it's practical to combine the two sites in one visit.
On 23 March 1944, the resitance group GAP killed 33 German soldiers with a bomb in Via Raselli. The Germans decided that 10 Italians were to be killed for each German soldier, and according to orders directly from Hitler, this was to be carried out within 24 hours. Pope Pius XII appealed on behalf of the victims, but in such a short time nothing could be done against an order from Berlin. 335 Italians were taken from various prisons and shot at the Ardeatine caves. They were buried at the spot and relatives often come here on Sundays. In March 1998, former SS officers Erich Priebke and Karl Hass were sentenced to life imprisonment in a Roman court for their roles in the massacre. The memorial is a strong reminder that persecution and inhumanity is not only something of the distant past.