Naples underground geothermal zone

Running beneath the Italian city of Naples and the surrounding area is an underground geothermal zone and several tunnels dug during the ages. This geothermal area is present generally from Mount Vesuvius beneath a wide area including Pompei, Herculaneum, and from the volcanic area of Campi Flegrei beneath Naples and over to Pozzuoli and the coastal Baia area. Mining and various infrastructure projects during several millennia have formed extensive caves and underground structures in the zone.

Source: Wikipedia