Naples, Italy at the base of Mount Vesuvius. Photodisc/ Getty Images

When it comes to Italy's Mount Vesuvius, ­it isn't a question of if it erupts but when. Ge­ologists and volcanologists who study the volcano readily concede that Mount Vesuvius is overdue for an explosion [source: Fraser]. For that reason, the Vesuvius Observatory monitors seismic activity, gas emissions and other indicators 24 hours a day to know at the earliest point when it may blow.

The infamous volcano is best known for its nearly instantan­eous decimation of neighboring towns Pompeii and Herculaneum in A.D. 79. Considered one of the world's most dangerous, it is also the only active volcano on Europe's mainland. Nevertheless, 600,000 people live in the 18 towns at its base that comprise the "red zone."



The red zone denotes the populated area that would bear the brunt of an ­eruption. Directly in the line of fire, the 9-mile (12-kilometer) radius of people stand little chance of survival when Vesuvius explodes again.

­Because of the imminent -- and unpredictable -- threat, the Italian government has devised an evacuation plan to clear out the red zone 72 hours ahead of an impending eruption. Beginning in 2004, the government also set up a program to pay people $46,000 (30,000E) to relocate outside of the zone -- though it has had relatively few takers. Experts warn that emergency plans should also include nearby Naples since an explosion could send dangerous burning ash and pumice as far as 12 miles (20 kilometers) [source: Fraser].

The last time Vesuvius activated was in 1944, causing minor damage and killing 26 people. New research has shown that the mountain probably will not act as kindly next time. For starters, Mount Vesuvius sits on top of a layer of magma deep in the earth that measures 154 square miles (400 square kilometers) [source: Noble]. That's a lot of magma -- Kilaeua Volcano is probably the most active volcano in the world, with 34 eruptions since 1952 [source: U.S. Geological Survey], but compared to Vesuvius, which has erupted around 30 times since 79 A.D. [source: Than], its magma supply is much smaller. Topping it off, scientists expect that the next eruption will be an incredibly forceful explosion, termed plinean, marked by flying rock and ash at speeds of up to almost 100 miles per hour (160 kph).

To summarize, if Mount Vesuvius erupts today, it wouldn't be a pretty picture. Given its potential, Vesuvius could endanger more than 3 million people and wipe out the city of Naples [source: Than].

Next, we'll learn what happened to Pompeii and Herculaneum that gave Vesuvius its notable reputation.