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Freak waves, also known as rogue waves, are legendary waves more than about twice the size of the largest typical waves. Until one was recorded by sensors on Draupner oil platform in the North Sea in 1995, it was thought that such waves may simply be a sailors' tall tale. Now it is known that freak waves are very much real, albeit extremely rare. They are so large, as tall as 25 m (27 yards) or more in height, that they pose a threat to ocean liners and other large ships. Because of their danger and the novelty associated with them, freak waves area a notable area of study among both oceanographers and physicists. To visualize the effect of a freak wave on an unfortunate ship, imagine a column of water the size of a 12-story building crashing down at the speed of a fast car.
Freak waves have long been part of the stories of sailors in older times. They told of waves so huge that they looked almost like vertical walls of water, and were proceeded by troughs so deep that they were veritable "holes in the sea." Such stories may have played a part in the foundation of myths of sea gods such as Poseidon. In modern times, statisticians approaching the problem used the Rayleigh probability distribution to measure the likely magnitude of waves, and found that waves with a height of over 15 m (16 yards) should be extremely rare. However, satellite imagery and ocean sensors since 1995 have invalidated this, showing that extraordinarily large waves do in fact form more frequently than theory would predict.
Scientists are still researching the likely causes of freak waves. One of the biggest risk factors is said to be when the area has a strong current running counter to the direction of the waves, such as certain areas around South Africa. Another causative factor is probably diffractive focusing - certain shapes in the seabed or along the coast can focus waves on a single point. There, constructive interference can occur - several smaller waves may all take place at the same place at the same time, building together into a massive freak wave. There are probably numerous other contributing factors, such as wind velocity, nonlinear effects, and focusing by current. Much more research and observation needs to be conducted before we truly understand the freak wave phenomenon.