The gigantic magnitude-9 earthquake that struck Japan on March 11, 2011, also unleashed an enormous tsunami that reached as high as 38 meters along the country's eastern coast. A seabed survey conducted off the Sanriku Coast in August 2011 discovered a surprising phenomenon. It is now being suggested that if widely expected earthquakes strike various regions of Japan in the future, this same phenomenon could produce more huge tsunami, and compound the damage wrought by these earthquakes. We introduce the newly understood mechanism behind mega-tsunami.