Since 1975, earthquakes have caused nearly 250 major tsunamis across the world. The following map depicts every event that the National Oceanic and Atmospheric Administration considers a "definite tsunami" —the highest value on a five-point scale of certainty. (It's not always easy to say whether a big wave should be classified as a tsunami. Along with earthquakes, gigantic waves can also be caused by things like landslides and volcanic eruptions.) The size and color of each dot is proportional to the magnitude of the quake that set off the wave. The tsunami that struck Japan last Friday is among the most severe in the past 35 years, and by far the worst to hit Japan in that time. Comparable events include a February 2010 tsunami in Chile, which stemmed from an 8.8 magnitude earthquake, and a tsunami caused by the 9.0-earthquake near Sumatra in December of 2004.