The coolest place in Orvieto (literally), this series of 440 caves (out of 1200 in the system) has been used for millennia by locals for various purposes – WWII bomb shelters, refrigerators, wine cellars, wells and, during many a pesky Roman or barbarian siege, as dovecotes to trap the usual one-course dinner: pigeon (still seen on local menus as palombo).
In fact, you only visit two caves (the rest remain in private hands – every home, shop etc in Orvieto has its own cave underneath). The one-hour tours (with English-speaking guides) leave from the office next to the tourist office on Piazza del Duomo.