The magnitude 7.9 quake was an oddity on its own. 'That's one of the things that makes this interesting and exciting and important to look into.' Rumbles from the deep Rare lower mantle quakes may be possible in particular conditions, says Heidi Houston, a geophysicist and deep quake expert at the University of Southern California who was not part of the study team. Seismic shudders are among the few windows into our planet's interior workings-and each unexpected happening, like a quake in the lower mantle, could offer new views into the underworld. While deep quakes don't cause the same kind of devastation as their shallow counterparts, studying these events can help scientists decipher the enigmatic ways our planet shifts far beneath our feet. The team detected several aftershocks around this depth-but one particular quake sat well beyond. Under Japan, the lower mantle is believed to start about 700 kilometres (435 miles) down.

While the boundary sits an average of 660 kilometres (410 miles) underground, it can vary around the globe. Some scientists caution that more research is needed to confirm the quake is real and did indeed strike in the lower mantle.

'This is by far the best evidence for an earthquake in the lower mantle,' says Douglas Wiens, a seismologist specialising in deep quakes at Washington University in St. While there have been hints of lower mantle quakes before, researchers have struggled to pinpoint them within this layer of the Earth.

The ultradeep quake, described recently in the journal Geophysical Research Letters, is estimated to have struck some 751 kilometers (467 miles) beneath the surface in the layer of our planet known as the lower mantle, where scientists have long thought earthquakes unlikely, if not impossible. Then another oddity emerged in the cascade of aftershocks that followed: a tiny tremor that, if confirmed, would be the deepest earthquake ever detected. The first jolt, which struck off the coasts of Japan's remote Bonin Islands, was recorded at magnitude 7.9 and up to 680 kilometres (423 miles) underground, making it one of the deepest quakes of its size. Most of Earth's tremors strike within a few dozen miles of the surface, but these quakes stirred at depths where temperatures and pressures grow so intense that rocks tend to bend rather than break. One spring evening six years ago, hundreds of miles underground, our planet began to rumble from a series of peculiar earthquakes.

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