When did catacombs start?

A:The Roman catacombs began as underground mines and quarries prior to the Common Era. When no longer in use, they were occupied and extended by the worshippers of Mithras, a Roman sun god, who needed to worship underground. Hundreds of temples, sculptures, fragments and inscriptions dedicated to Mithras have been found in the catacombs, including the main large Mithraic temple that lies directly underneath the Vatican. However, there are also images in the catacombs of the Egyptian god Horus as a baby being held by the Virgin Isis-Meri.

In the third century CE, with Mithraism losing support, Christianity was strong enough to appropriate the Mithraic catacombs for its own use. It is believed that the Christians began to use the catacombs for Christian burials as an act of impiety towards Mithraism.