The immense swell of a tsunami can grow up to 100 feet, hitting speeds over 500 mph -- a treacherous combination for anyone or anything in its path. Tsunamis can be compared to normal waves in the sense that they are energy moving through water. However, in normal waves this energy comes from wind, which only moves across the surface of the water. In tsunamis, the energy comes from an underwater earthquake, volcanic eruption, or submarine landslide. In this case, an enormous amount of energy is displaced and makes the sea level rise, only to have gravity bring it back down, thus creating a ripple outwards horizontally. Hence you can see that although tsunamis are sometimes called tidal waves, they actually are not related to tides at all.
When a tsunami is far from shore, sometimes it can be barely visible, because it moves throughout the depths of the ocean. Yet when it reaches shallower waters, wave shoaling occurs. This means that waves are compressed and the height of the tsunami grows substantially. If the trough of the wave hits the shore first, the wave can retreat in a dangerously misleading way and then hit shore suddenly and intensely. Tsunamis have caused destruction around the world because of their unpredictability and ability to affect those living up to a mile inland of the ocean.