The world's oceans claim on average one ship a week, often in mysterious circumstances. With little evidence to go on, investigators usually point at human error or poor maintenance but an alarming series of disappearances and near-sinkings, including world-class vessels with unblemished track records, has prompted the search for a more sinister cause and renewed belief in a maritime myth: the wall of water. Waves the height of an office block. Waves twice as large as any that ships are designed to ride over.These are not tsunamis or tidal waves, but huge breaking walls of water that come out of the blue.

Since 1990, 20 vessels have been struck by waves off the South African coast that defy widely accepted wave model's predictions. And on New Year's Day, 1985 a wave of 26m was measured hitting the Draupner oil rig in the North Sea off Norway. Al Osborne, a wave mathematician with 30 years experience devising equations to describe open ocean wave patterns, suggests that there are two kinds of waves out on the high seas; the classical undulating type described by the linear model and an unstable non-linear monster - a wave that at any time can start sucking up energy from waves around it to become a towering freak.

The consequences for ship design could be stark. Currently the biggest wave factored into most ship design is smooth, undulating and 15m high. A freak wave is not only far bigger, it is so steep it is almost breaking. This near-vertical wall of water is almost impossible to ride over - the wave just breaks over the ship. According to accident investigator, Rod Rainey, such a wave would exert a pressure of 100 tonnes per square metre on a ship, far greater than the 15 tonnes that ships are designed to withstand without damage. If the current assumptions were to be proved false, the whole world shipping industry would face some very tough choices.