They are sometimes mistakenly called tidal waves. Tsunamis have nothing to do with tides. They are caused by massive underwater earthquakes. If the earthquake happens far out to sea, tsunamis are hardly noticeable. They are like giant waves that ride on the ocean floor and can travel up to 500 mph. When the ocean floor rises up to meet the shoreline or harbor, thats when this huge wave suddenly appears. The wave may only be noticeable for about a mile out before it hits the shore.