A sneaker wave, or sleeper wave, is an unusually large wave that can appear within a train of smaller waves without warning. The term is popular rather than scientific. NOAA loosely defines sneaker waves as waves near shore that are unexpectedly and significantly larger than other waves arriving at the same time.
Researchers suggest they come from distant storms that send long-period waves, which then arrive during calm weather and run farther up the beach than expected. Along the U.S. West Coast, sneaker waves kill more people than all other weather hazards combined.
On February 13, 2010, two large waves suddenly struck a crowd watching the Mavericks surf contest from a narrow beach, breaking over a seawall and injuring at least 13 people.
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