A volcano is a sort of giant chimney for the inside of the Earth. Deep down, the rock is so hot it has melted. Where two parts of the Earth's crust meet, that hot liquid sometimes finds a way to the surface. When it comes out, we call it lava.
Mount Aso, on the island of Kyushu, is one of Japan's biggest active volcanoes. Its crater is enormous - over 25 km across. Towns and farms sit inside the crater, and you can stand at the edge and watch a smaller volcano in the middle puff out steam.
Japan's volcanoes have given the country one of its best-loved things: onsen, the natural hot springs. Underground water heated by the volcanoes bubbles up to the surface at the perfect temperature for a warm bath. There are thousands of onsen around the country. Even the snow monkeys use them.
Living near volcanoes means Japan also gets earthquakes. Japanese children practise earthquake drills at school, the same way other countries practise fire drills. Many buildings are specially designed to wobble and bend instead of breaking when the ground shakes.