They are creatures of the open ocean, trains of giant waves that can travel for thousands of miles across the sea and still pack enough energy to smash towns and drown the unwary. A tsunami generated in the Aleutian Islands struck Hawaii in 1946. Toss a stone in a pond and you create a series of concentric ripples. A tsunami is just like those ripples, except the disturbance that sets them in motion is of a much greater magnitude. Undersea landslides and the collapse of oceanic islands into the sea make tsunamis.
Volcanic eruptions can also do it.
In fact, the most deadly tsunami in recorded history followed the eruption and virtual obliteration of Indonesia's Krakatoa Volcano in 1883. An estimated 36,000 people died as a result of the eruption, the majority of them from the tsunamis. Son of Krakatoa. Impacts by comets or asteroids can also generate giant tsunamis. Scientists at the Los Alamos National Laboratory in New Mexico calculated that if an asteroid three miles across hit the middle of the Atlantic Ocean, the tsunami would swamp the upper East Coast as far inland as the Appalachian Mountains and drown the coasts of France and Portugal.