Tsunamis are very long waves in the ocean, sometimes hundreds of miles long. Tsunamis usually start suddenly. They may begin as normal waves and change to big waves very quickly. After this change, tsunami waves will travel at great speed across the ocean with little energy loss. Just before they hit land, the water will draw back off of the coast. If the slope of the coast is shallow, the water may pull back for many hundreds of feet. People who are unaware of the danger may be drawn by this strange site and remain on the shore. When the tsunami finally hits, it may remove sand from beaches, destroy trees, damage houses and even destroy whole towns. Tsunamis are tremendously powerful.