Extending beneath the Basilica di San Sebastiano, these underground burial chambers were the first to be called catacombs – the name was derived from the Greek kata (near) and kymbas (cavity), because they were located near a cave. They were heavily developed from the 1st century and during the persecutory reign of Vespasian they provided a safe haven for the remains of Saints Peter and Paul.

The 1st level is now almost completely destroyed, but frescoes, stucco work and epigraphs can be seen on the 2nd level. There are also three perfectly preserved mausoleums and a plastered wall with hundreds of invocations to Peter and Paul, engraved by worshippers in the 3rd and 4th centuries.

Above the catacombs, the basilica, a much-altered 4th-century church, preserves one of the arrows allegedly used to kill St Sebastian, and the column to which he was tied.