On March 11, 2011 the Great East Japan Earthquake struck off the Sanriku coast, registering a magnitude 9, the largest in the history of observations in Japan. The earthquake was accompanied by extremely powerful tsunami waves with the maximum run-up height of 40.5 meters. This disaster caused the death or disappearance of almost 20,000 people, with 95% of the victims being swept away by the tsunami. In order that we never incur such tsunami related casualties again, it is most important that we first acquire a correct understanding of the true nature of tsunamis.