Matera is a fairly big city. One of its quarters is the old city. Just outside the old city are the Sassi - cave houses dug into the limestone rocks. The sassi are located on both sides of a valley. On the north side the Sasso Barisano have been partially commercially developed as offices and housing. The Sasso Caveoso, on the south side, are the most historically interesting. http://wikitravel.org/en/Matera