What exactly are the catacombs?

They are subterranean cemeteries created by the early Christians to bury their dead. They are primarily located outside of the city walls of ancient Rome.

They were created by digging out a very soft, porous sedimentary rock called tuffa. Before its exposed to air, tuffa rock is very soft and relatively easy to excavate by hand tools. However, after it has been exposed to air, it gradually hardens. So the passageways carved out become firm instead of caving in.

The word “Catacombs"

Most Christians are surprised when I tell them that there is no mention of the catacombs in the writings of the early Christians. That's because the early Christians didn't refer to these burial places as the “catacombs.” They simply called them “cemeteries.”

You're no doubt wondering: “So where did the word “catacombs” come from?” It originally was simply a geographic term—not having anything whatsoever to do with the early Christian cemeteries. Ancient maps carried the notation, “ad catacumbas,” for an area around the Appian Way where the land dipped down—where there were hollows. Ad catacumbas is simply Latin for “near the hollow.” The name for the region was there before the early Christians built their underground burial chambers.

Now, not too far from the Catacomb of St. Callistus, there is another underground cemetery named after a saintly Christian called Sebastian. Well, in the late 4th and the 5th centuries, a lot of pilgrims came to Rome to view these underground burial chambers. And maps and guides were made for these pilgrims. In these guides and other documents, the Sebastian cemetery was given this name: “Cymiterium Catcumbas ad sanctum Sebastianum via Appia.” This name was merely giving the location of this cemetery—being one of the cemeteries located in the catacumbas region along the Appian Way.

Somehow, during the Middle Ages—primarily through ignorance, people started referring to all of these underground cemeteries as “catacombs.” And that's how the name got started.