The Sassi — from the Italian word for "stones" — are the cave districts that line two ravines below the Civita, Matera's original hilltop nucleus. They are divided into the Sasso Caveoso (south, older and rougher) and the Sasso Barisano (north, more restored). Together they constitute one of the most extraordinary urban landscapes in the Mediterranean: roughly 9,000 cave dwellings, rock-cut churches, and interconnected cisterns dug into the soft tufa over a period stretching from the Palaeolithic to the 20th century.
The cave churches are the most remarkable element — there are over 150 in the ravines and on the Murgia plateau across the canyon, decorated by Byzantine monks between the 8th and 13th centuries. The frescoes vary dramatically in quality and preservation; the tourist office in Piazza Vittorio Veneto can advise on which are open and which require advance permission.
Walking the Sassi requires comfortable shoes and some stamina: the alleys climb and drop without warning, and many of the most interesting sections are away from the main tourist paths. The best overview is from the belvedere at the top of the Civita, which gives you the full sweep of both ravines and the Murgia tableland behind.