On 30 May 2015, a magnitude 7.9 earthquake took place beneath Japan's remote Ogasawara (Bonin) Islands, located about 1,000 kilometers south of Tokyo. The seismic activity occurred over 660 kilometers below Earth's surface, near the transition between the upper and lower mantle. The mechanism of deep-focus earthquakes, like the 2015 quake, has long been mysterious—the extremely high pressure and temperature at these depths should result in rocks deforming, rather than fracturing as in shallower earthquakes.