"As the tsunami enters the shoaling water near the coast, its velocity decreases and its height increases. It is in these shallow waters that tsunamis become a threat to life and property, for they can crest to heights of more than 30-50 meters and strike with devastating force.

Tsunamis are among the most terrifying natural hazards known to man. They have been responsible for tremendous loss of life and property throughout history."

Citation source: http://www.drgeorgepc.com/TsunamiFAQ.html

Source of picture: http://www.spiegel.de/fotostrecke/0,5538,4807,00.html