Rogue waves (also known as freak waves, monster waves, killer waves, extreme waves, and abnormal waves) are relatively large and spontaneous surface waves that occur far out in open water, and are a threat even to large ships and ocean liners.They present two kinds of danger: although rare, they are unpredictable, and may appear suddenly or without warning, and they can impact with tremendous force (a 12 meter wave in the usual ""linear"" model would have a breaking force of 6 million tons per square metre (MT/m2); modern ships are designed to tolerate a breaking wave of 15 MT/m2), but a rogue wave can dwarf both of these figures with a breaking force of 100 MT/m2.In oceanography, rogue waves are more precisely defined as waves whose height is more than twice the significant wave height (Hs or SWH), which is itself defined as the mean of the largest third of waves in a wave record. Therefore, rogue waves are not necessarily the biggest waves found on the water; they are, rather, unusually large waves for a given sea state. Rogue waves seem not to have a single distinct cause, but occur where physical factors such as high winds and strong currents cause waves to merge to create a single exceptionally large wave.Rogue waves can occur in other media than water. In particular, optical rogue waves allow study of the phenomenon in the laboratory. A 2015 paper studied the wave behavior around a rogue wave, including optical, and the Draupner wave, and concluded that ""rogue events do not necessarily appear without a warning, but are often preceded by a short phase of relative order"".