How do tsunamis become so much taller when they reach the shore?

Tsunamis are usually caused by sudden seafloor movement, often from undersea earthquakes, landslides, or volcanic collapses. In deep ocean water, they can travel at jet-plane speeds, but with low wave height and very long wavelength. As the wave reaches shallow water, it slows down and its energy is forced upward, making the wave grow taller. The 2004 Indian Ocean tsunami and the 2011 Tōhoku tsunami in Japan showed how devastating these waves can be. Some tsunamis also arrive as a series of waves rather than a single crest, which is why people are warned not to return too soon.