A volcano is a kind of mountain with a passage that goes deep down to the hot middle of the Earth. The reason Indonesia has so many is that it sits where two giant pieces of the Earth's surface, called tectonic plates, meet. When two plates push against each other, the rock down below gets hot, and sometimes a path opens up and the heat reaches the surface.
Indonesia is part of a larger ring of volcanoes that goes all the way around the Pacific Ocean. Scientists call it the Pacific Ring of Fire - even though there isn't actually any fire, just very, very hot rock far below.
Living near a volcano sounds strange, but it has a wonderful side. Old volcano ash is one of the richest soils on Earth - full of minerals plants love. That is why farms in places like Java grow such healthy rice, vegetables and coffee. Some of the most beautiful rice fields in the world climb the slopes of old volcanoes.
One of the most famous volcanoes in Indonesia is Mount Bromo on the island of Java. People wake up before dawn to watch the sun rise over its misty crater, while smaller cones around it sit like a row of sleeping giants. Indonesia's tallest volcano, Mount Kerinci, is 3,805 metres - higher than most ski lifts in the Alps.