Phlegraean Fields (Italy)
This sprawling caldera near Naples shares dangers with nearby Campi Flegrei. Yellow sulfur pools and cracked earth reveal its restless nature. The last eruption in 1538 created a new mountain overnight. Recent ground uplift of 6 feet and frequent earthquakes keep half a million residents concerned. Fumaroles release volcanic gases through urban neighborhoods. Ancient Romans believed this area was the entrance to the underworld.