Use the noun catacomb to talk about an old underground cemetery. The most famous catacombs were built by the ancient Romans.
You're most likely to come across this word in its plural form, catacombs, and it's almost always used in the context of the ancient Roman empire. A catacomb is shaped like a long tunnel, with space for bodies to be buried, often in tombs. The root word is the Late Latin catacumbae, which refers to one specific catacomb under the Appian Way in ancient Rome, in which the Biblical apostles Peter and Paul are said to be buried.