A wave twice as high as the sea around it was measured

On 1 January 1995 a laser on the Draupner platform recorded a wave 25.6 metres high, in a sea whose significant wave height was about 12 metres. It was the first rogue wave caught by an instrument.

The significant wave height is a statistical description of the sea state, roughly the average of the highest third of the waves. Linear theory expects a single wave to stray only so far above that average, and a wave more than twice the significant height is, on that theory, a freak — possible, and so rare that many engineers treated the stories as exaggeration. Draupner’s downward-looking laser, confirmed by other sensors on the jacket, recorded a crest-to-trough height of 25.6 metres while the significant height was near 12. The crest alone stood 18.5 metres above the still-water line. The platform, built for a 20-metre design wave, took minor damage. Nobody died.

One record ended the argument about existence and started the argument about cause. Crossing seas, where two wave trains meet at an angle, can focus energy into a single steep crest, and later tank experiments have rebuilt a Draupner-like profile that way. The cause can be debated. The tape cannot. The sea really does, sometimes, stand up twice as high as the sea it is part of, and it does it in seconds, and then it is gone.