Have you ever thrown a big stone into a pond? The stone makes waves. The water in the ocean moves in waves, too. One kind of wave called a tsunami is dangerous. It starts when big pieces of land move lots of water quickly. The islands of Indonesia are made of many volcanoes. When volcanoes erupt, they can start tsunamis. On December 22nd, 2018, the volcano on Anak Krakatau erupted. The walls of the mountain fell into the sea. That started a huge tsunami.