How can they be similar? Surfing is done on water; skiing on snow. The main similarity is that in both cases the athlete and board travel down a hill. In skiing, the hill is a mountain covered with snow, while in surfing the hill is the rising water of a breaking ocean wave. An ideal surfing wave has a large amplitude as it reaches an extremely gradual decrease in ocean depth. While the surfer moves down the wave, the water on the front edge of a crest continually rises underneath the surfer, allowing the surfer to ride down the wave without actually moving downward.

A tidal wave, or tsunami, is not caused by windy conditions or tides, but instead by underwater earthquakes and volcanic eruptions. The seismic disturbances create huge upward forces on the water, the opposite of dropping rocks into water. A tsunami is a series of several waves with a period of more than 30 minutes between each crest. The ocean first recedes from the beach, then water rushes inland at a very high speed. Large tsunamis can be quite destructive upon reaching the shoreline due to their amplitudes.

The most deadly recorded tsunami occurred December 26, 2004, in the Indian Ocean. The earthquake that caused it was off the west coast of Sumatra, Indonesia. That quake released an amount of energy equivalent to 550 million times the energy released in the Hiroshima nuclear bomb. One part of the ocean bottom was lifted by 4 to 5 meters (13 to 16 feet) and moved horizontally 10 meters (33 feet). The tsunami, traveling at a speed of 500 to 1,000 kilometers per hour, had a low amplitude (60 centimeters) in mid-ocean, but when it crashed into the coasts from Thailand to India and as far as South Africa it had an amplitude as high as 24 meters (79 feet). Some 230,000 people were killed and more than a million made homeless.