Mount Vesuvius was a volcano in Italy. It was famous for its major eruption in 79 AD where it buried Pompeii under several feet of volcanic rock. It last erupted in 1944.
During the 2012 apocalypse, when the Earth's crust became unstable, Mount Vesuvius presumably erupted again, which destroyed the remains of the ruined Pompeii and the city of Naples. Pyroclastic flows began rendering much of Italy uninhabitable, along with Mount Etna which also erupted during the apocalypse. Mount Vesuvius was later swept by a megatsunami during the Worldwide Flooding.
When the apocalypse ended, Mount Vesuvius is submerged underwater and is left uninhabitable while becoming dormant again.