Mt. Vesuvius in Italy and Mt. St. Helens in the United States are both famous mountains. They are both volcanoes. A volcano is a mountain that is open at the top. Smoke and hot air come out of the hole. Sometimes very hot rock also comes out of the mountain. That can mean trouble for people nearby. This is what happened with Mt. Vesuvius and Mt. St. Helen. Hot rock poured out of Mt. Vesuvius and cover the town of Pompeii in 79 AD. Everyone in the town was killed. The Mt. St. Helens volcano did not kill many people. There were no cities close to the mountain. But the hot rock killed a large part of the forest. And a lot of dirt fell on cities many miles away.