Rogue, freak, or killer waves have been part of marine folklore for centuries, but have only been accepted as a real phenomenon by scientists over the past few decades.
Rogues, called 'extreme storm waves' by scientists, are those waves which are greater than twice the size of surrounding waves, are very unpredictable, and often come unexpectedly from directions other than prevailing wind and waves.
Most reports of extreme storm waves say they look like "walls of water." They are often steep-sided with unusually deep troughs.
Since these waves are uncommon, measurements and analysis of this phenomenom is extremely rare. Exactly how and when rogue waves form is still under investigation, but there are several known causes:
Constructive interference. Extreme waves often form because swells, while traveling across the ocean, do so at different speeds and directions. As these swells pass through one another, their crests, troughs, and lengths sometimes coincide and reinforce each other. This process can form unusually large, towering waves that quickly disappear. If the swells are travelling in the same direction, these mountainous waves may last for several minutes before subsiding.
Focusing of wave energy. When waves formed by a storm develop in a water current against the normal wave direction, an interaction can take place which results in a shortening of the wave frequency. This can cause the waves to dynamically join together, forming very big 'rogue' waves. The currents where these are sometimes seen are the Gulf Stream and Agulhas current. Extreme waves developed in this fashion tend to be longer lived.