I have gotten some comments and questions about this blog post. Here's what I found on Wikipedia about these catacombs:
Catacombs of Domitilla
Close to the Catacombs of San Callisto are the large and impressive Catacombs of Domitilla (named after Saint Domitilla), spread over 15 kilometers of underground caves. Entrance is through a sunken 4th-century church, at via delle Sette Chiese 280. Those catacombs are oldest of Rome's underground burial networks, and the only one to still contain bones. Catacombs include 2nd-century fresco of the Last Supper and other valuable artifacts.
In the beginning of 2009, at the request of the Vatican, Divine Word Missionaries––an Roman Catholic Society of priests and Brothers––assumed responsibility as administrator of St. Domitilla Catacombs.
The practice of burying the dead underground began in the 2nd century. The soft volcanic rock under Rome makes tunneling quite easy, and when the material is exposed to air it becomes very hard. In and around Rome there are miles of these underground burial caves built between the 2nd and 9th centuries.
The Catacombs of St. Domitilla are the best preserved and one of the most extensive of all the catacombs. This catacomb is also unique in that it has a subterranean basilica. The basilica eventually became unsafe and it was abandoned in the 9th century. It was rediscovered in 1593, and much of it was reconstructed in 1870.