Tidal waves, or tsunami, as scientists call them, are very often the result of earthquakes on the bottom of the ocean. The sea bed shifts and slides like an earthquake on land, causing a great shock wave. Ships that are in the area of the quake fell this shock wave as if they had just struck a rock. Sometimes the shock causes a great depression in the water, and sometimes an enormous mound of water is builds up and travels with great speed across the ocean. Volcanoes can also cause tidal waves. In 1883 the island of Krakatoa in the Dutch East Indies was almost destroyed by a huge volcanic eruption. Great tidal waves were caused by the shock that this explosion started, and the waves made themselves felt on the coasts of Australia and California thousands of miles away.
There are warnings signs on land which tell us that tidal wave is approaching. Firstly, there is a swell like an ordinary wave. Then the sea level falls for some time, as though it were very low tide, and exposes a large area of the sea bed around the coast. Ten the tidal wave, sometimes as high as two houses, comes crashing into the shore.