Mount Vesuvius in southern Italy is actually a volcano inside the exploded skeleton of an older volcano. Looked at from above, the remaining ridge of a much larger volcano can be seen on the north side. This older volcano had probably erupted violently long before human settlement. Southern Italy is unstable ground. The African continental plate, on which most of the Mediterranean Sea rests, is actually diving beneath the European plate. That kind of underground collision produces molten rock, or magma, rich in volatile gases such as sulfur dioxide. Under pressure underground, these gases stay dissolved. But when the magma rises to the surface, the gases are released. Accordingly, when volcanoes like Vesuvius erupt, they tend to erupt explosively. To this day, in fact, Vesuvius remains one of the world’s most dangerous volcanoes; some 3.5 million Italians live in its shadow. Although monitoring devices are in place to warn of the volcano’s activity, if there were a major eruption with little warning, there could be a tremendous loss of life.