Although, for a given location on the Earth's surface, the risk of a "direct" hit from an asteroid is slight, researchers realized that an ocean impact had the potential to be much more destructive due to the effects of tsunami. An airburst explosion is a three dimensional event and energy decreases according to the square of the distance but a radiating ocean wave is a two-dimensional phenomenon and, in theory, energy decreases in proportion to distance. Since the early 1990s some advanced computer simulations have been conducted to estimate the effects of asteroid impacts above deep oceans.
The dramatic picture by Don Davis is a little misleading. When an asteroid hits the ocean at 70 000km/h there is a gigantic explosion. The asteroid and water vaporize and leave a huge crater - typically 20 times the diameter of the asteroid (that is, a 100m asteroid will create a 2 kilometre diameter crater). The water rushes back in, overshoots to create a mountain of water at the middle and this spreads out as a massive wave - a tsunami. The centre of the "crater" oscillates up and down several times and a series of waves radiate out. An idea of the mechanism can be obtained by bursting a balloon in a bathtub.
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