Rogue waves are also known as freak waves or monster waves. These kinds of waves are relatively large and spontaneous ocean surface waves which can sink even medium-large ships. They are now known to be a natural (although relatively rare) ocean phenomenon. In certain areas the probability of encountering large waves is much greater than in other areas. For example, in the Agulhas current outside South Africa, many large ships have encountered difficulties with rogue waves, because the strong current going south meets strong swell from storms in the Antarctic Ocean.

A rogue wave can reach a height of 30 m, but computer simulation programs can predict rogue waves of more than that.