Tsunami

An ocean wave formed by seismic activity on the sea floor that can produce dangerously large and powerful breaking waves along certain coastlines. "Tsunami" is the modern word for a "tidal wave", but that term was deemed in accurate as the waves have nothing to do with tide (although they can look like one). The word comes from Japanese. The literal translated meaning is "tidal wave" so they didn't really fix anything.

As a deep water wave, a Tsunami will actually only have a wave height of only a few inches and is almost undetectable in the open ocean. But it has a long wavelength and extrememly high wave velocity and contains an enormous amount of energy so when it gets into shallow water the energy can empty bays and innundate low lying towns.

If your normally deep harbor suddenly empties out, get to high ground fast - the second part of the wave is coming fast.