Tsunamis

A tsunami can be triggered by for example an earthquake, volcanic eruption or landslide. Around 80% of all tsunamis are caused by earthquakes. The tsunami is triggered when the seafloor is abruptly deformed and the water above it is vertically displaced. A wave with a low wave height (around half a meter) and long wavelength (more than 10 km) is created and travels over the sea. When the wave reaches the shore, it compresses and transfers to a high wave with short wave length (read more).

And for all my surfer crazy friends, from the GNS Science GNS Science website:

"Is a tsunami a dream come true for extreme surfers?
No. A tsunami is not a wave in the classical sense, but a raging torrent of water that surges inland with enormous power - much more power than a surfer can handle. Besides the waves do not stop at the shore, so surfers may find that they crash into buildings or all the other debris caught up in the surging water. Invariably a tsunami consists of successive surges or torrents and equally violent return flow to the sea. The first wave or torrent of a tsunami is not necessarily the biggest. The second or third or even much later waves may be bigger. Intervals between successive waves can vary. It may just be minutes, or it could be more than an hour."