How do tsunamis travel across the ocean so fast?

A tsunami is usually triggered by an undersea earthquake, but volcanic eruptions, landslides, or meteor impacts can also cause one. In the deep ocean, a tsunami may be only a few tens of centimetres high, so ships often do not notice it. Its speed depends on water depth, and in deep water it can travel as fast as a commercial airliner. When it reaches shallow coastal water, it slows down and grows much taller, which is why it can become devastating near shore. The 2004 Indian Ocean tsunami and the 2011 Tōhoku tsunami in Japan showed how destructive these waves can be.