One spring evening six years ago, hundreds of miles underground, our planet began to rumble from a series of peculiar earthquakes. Most of Earth's temblors 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.

The first jolt, which struck off the coasts of Japan's remote Bonin Islands, was recorded at magnitude 7.9 and up to 680 kilometers (423 miles) underground, making it one of the deepest quakes of its size. Then another oddity emerged in the cascade of aftershocks that followed: a tiny temblor that, if confirmed, would be the deepest earthquake ever detected.

The ultradeep quake, described recently