A sneaker wave, also known as a sleeper wave, or in Australia as a king wave, is a disproportionately large coastal wave that can sometimes appear in a wave train without warning. Terminology The term "sneaker wave" is popular rather than scientific, derived from the observation that such a wave can "sneak up" on an unwary beachgoer. There is no scientific coverage of the phenomenon as a distinct sort of wave with respect to height or predictability as there is on other extreme wave events such as tsunamis or rogue waves, and little or no scientific evidence has been gathered to identify, describe, or define sneaker waves. Although the term "rogue wave" — meaning an unusually tall or steep wave in mid-ocean — is sometimes used as a synonym for "sneaker wave". One American oceanographer distinguishes "rogue waves" as occurring on the ocean and "sneaker waves" as occurring at the shore, while the National Oceanic and Atmospheric Administration loosely defines rogue waves as offshore waves that are at least twice the height of surrounding waves and sneaker waves as waves near shore that are unexpectedly and significantly larger than other waves reaching shore at the time. Scientists do not yet understand what causes sneaker waves, and their relationship to rogue waves, if any, has not been established. In a 2018 paper, Oregon State University researchers wrote that sneaker waves form in offshore storms that transfer wind energy to the ocean surface. The resulting waves then arrive along a coastline during periods of calm weather, and the greater amount of energy they contain compared to the regular waves that preceded them causes them to travel far higher up the shore than the other waves. As of 2021, the National Weather Service in the United States viewed ocean conditions along the United States West Coast as favorable for sneaker waves when an offshore storm generates waves with a particularly long period — perhaps longer than 15 seconds — between swells, allowing the swells to build considerable force before reaching shore, where they might appear either as conventional large waves or as sneaker waves. Characteristics Sneaker waves appear suddenly on a coastline and without warning; generally, it is not obvious that they are larger than other waves until they break and suddenly surge up a beach. A sneaker wave can occur following a period of 10 to 20 minutes of gentle, lapping waves.