Tsunamis are caused by sudden shifts in the seafloor, often after a strong underwater earthquake.

The longer version

Vertical movement of the seabed displaces the full water column above it, producing a wave with an enormous wavelength and open-ocean speed comparable to a fast aircraft. In deep water the crest may pass unnoticed beneath a vessel.

Approaching land changes everything: the leading edge slows in shallower water while the rest still races forward, so energy crowds into a shorter, far taller wave. Drawback is often the first sign. Reefs, bays and coastal shape then decide how high the water actually climbs at any particular spot. The same mechanism follows underwater landslides and volcanic collapse, though far less often and usually on a smaller scale.

Where this entry sits

Oceans and Sea Life looks at the salt water that covers most of the planet, from crashing waves and tides to the darkest deep-sea trenches. Expect notes on coral, fish, whales, sharks, jellyfish, shells, and the strange creatures that live far below the surface. The full list sits on the Oceans and Sea Life page, where every entry is listed in order.