Remarkably, this now-atmospheric, vaulted space was filled with debris until 1999 (with the rubbish having been deposited in the 1300s). Now you'll see 180 sq metres of 13th century pintura a secco ; these 'dry paintings' (also called 'mural paintings') contrast with frescoes which are painted on wet plaster, making them more durable. Buy tickets from the duomo ticket office.
