BBC - Earth - Terrifying 20m-tall 'rogue waves' are actually real:
"...A rogue wave is one that is at least twice the "significant wave height", which refers to the average of the third highest waves in a given period of time.
According to satellite-based measurements, rogue waves do not only exist, they are relatively frequent.
...At 3am on 12 December 1978, a German cargo ship called The München sent out a mayday message from the mid-Atlantic.
Despite extensive rescue efforts, she vanished never to be found, with the loss of 27 lives.
A lifeboat was recovered.
Despite having been stowed 66ft (20m) above the water line and showing no signs of having been purposefully lowered, the lifeboat seemed to have been hit by an extreme force.
However, what really turned the field upside down was a wave that crashed into the Draupner oil platform off the coast of Norway shortly after 3.20pm on New Year's Day 1995.
Hurricane winds were blowing and 39ft (12m) waves were hitting the rig, so the workers had been ordered indoors.
No-one saw the wave, but it was recorded by a laser-based rangefinder and measured 85ft (26m) from trough to peak.
The significant wave height was 35.4ft (10.8m).
According to existing assumptions, such a wave was possible only once every 10,000 years..."