The city is known worldwide for its historic Sassi districts, which make it one of the oldest inhabited cities in the world. The Sassi were recognized in 1993 as a UNESCO heritage site. In 2019, Matera was the European Capital of Culture. The Gravina stream, a tributary of the Bradano, flows in the deep natural ditch that delimits the two ancient districts of the city: Sasso Barisano and Sasso Caveoso. On the other bank is the Murgia, part of the Murgia Materana Park (the Regional Archaeological Historical Natural Park of the Rupestrian Churches). The ancient Sassi districts, together with the cisterns and water collection systems, are the peculiar characteristic of Matera.