Beneath the newer parts of Sousse lie over 240 galleries of Roman-era Christian catacombs — one of the most extensive in the world outside Rome. The tunnels were cut into the soft limestone between the 2nd and 4th centuries CE, as the Christian community of Hadrumetum grew and needed burial space outside the city walls. The catacombs contain around 15,000 graves.
Only a portion of the galleries is open to visitors, but the section that can be toured gives a clear sense of the scale. The walls are lined with loculi — horizontal niches cut for individual burials — and occasional larger arched chambers for wealthier families. Some sections have mosaic floor fragments and painted inscriptions. The catacombs are reached through the museum in the Kasbah area; ask at the ticket desk for access.