When the magnitude-9 Tohoku earthquake hit Japan on March 11, 2011, the mainshock triggered tsunami waves averaging about 10 meters in height by the time they reached the coast of Japan, from Fukushima in the south to the northern tip of Honshu Island. But one mountainous stretch of coastline known as Sanriku, about 100 kilometers north of the main rupture area, saw waves higher than 40 meters. This oddity has led some scientists to suggest that a submarine landslide, triggered by the earthquake, may have contributed to the tsunami’s extreme height in this region.