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.

Mining and various infrastructure projects during several millennia have formed extensive caves and underground structures in the zone.

Tuff was mined through access and removal shafts called the occhio di monte, ("eye of the mountain").

The resulting void was a bottle shaped cavity with sloping shoulders, which provided ample reinforcement to prevent future cave-ins.

In World War II, many of the quarry shafts were enlarged and spiraling stairways were added, opening up the caverns for use as air raid shelters.