In 79 AD, 20,000 Romans lived in the town of Pompeii, near present-day Naples. Many were farmers, working the fertile soil that, unbeknownst to them, came from previous eruptions of nearby Mount Vesuvius.

Then in August, the top of that mountain exploded 10 miles into the atmosphere! Ash and stone rained down on Pompeii for a day. Most residents fled.

The 2,000 who stayed thought they had escaped the worst of it—then a cloud of hot gas rolled down the mountain, suffocating them. Their bodies were encased in ash, to be found almost 2,000 years later.

There are two main types of lava, and Vesuvius has the more dangerous kind. It’s high in silica, making it very viscous, or thick, so that it traps the gases that were responsible both for the mountain’s explosion and its deadly clouds.

Conversely, Hawaii’s Kilauea has been continuously erupting since 1983. Its lava is high in iron and magnesium, flows easily, and therefore traps less gas, making it much less explosive. Scientists can walk right up to it and sample lava.

Since 79 AD, Vesuvius has erupted 30 more times. It remains the only active volcano in Europe.

Scientists know that it will erupt again but can’t be sure when or how dramatic it will be.

Some expect soon, and they’re keeping a close eye on this lightly sleeping giant.