The Mayor's Vault (Voltone del Podestà) opens under the building that bears the same name in Piazza Maggiore. It is included among the seven secrets of Bologna for the peculiar sound game that is created under its vault. Visitors will experience a “wireless telephone” by talking to each other from one corner of the structure to another.
Originally the headquarters of the market and of the notaries' stalls, it was later used as a place to administer public reprimand to blasphemers or even death sentences by hanging. A few centuries later a religious significance was also attributed to the vault with the placement of the four statues of the protectors of Bologna: San Petronio, San Francesco, San Domenico and San Procolo.