Move aside, Bermuda Triangle: The newest North Atlantic mystery lies beneath this enigmatic archipelago. Scientists have discovered a strange, 12.4-mile-thick (20 kilometers) rock layer below the oceanic crust under Bermuda. This level of thickness has never been seen in any other similar layer worldwide.

“Typically, you have the bottom of the oceanic crust and then it would be expected to be the mantle,” said study lead author William Frazer, a seismologist at Carnegie Science in Washington D.C. “But in Bermuda, there is this other layer that is emplaced beneath the crust, within the tectonic plate that Bermuda sits on.”

While the origin of this layer is not entirely clear, it may explain an ongoing mystery about Bermuda, Frazer told Live Science. The island sits on an oceanic swell, where the ocean crust is higher than its surroundings. But there is no evidence of any ongoing volcanic activity creating that swell — the island’s last known volcanic eruption was 31 million years ago.

The discovery of the new giant “structure” suggests the last eruption may have injected mantle rock into the crust, where it froze in place, creating something like a raft that raises the ocean floor by about 1,640 feet (500 meters). Bermuda has long had a reputation for mystery, largely because of the Bermuda Triangle, an area between the archipelago, Florida and Puerto Rico where a supposedly unusual number of ships and aircraft have gone missing. (This reputation, however, has been largely exaggerated.) The real mystery, though, is why the Bermuda oceanic swell exists.

Island chains such as Hawaii are thought to exist because of mantle hotspots, which are places in the mantle where hot material rises, creating volcanic activity. At the point where the hotspot meets the crust, the ocean floor often buoys up. But when tectonic movement slides the crust away from that hotspot, the oceanic swell typically subsides.

Bermuda’s swell hasn’t subsided, despite 31 million years of volcanic inactivity there, Frazer said. There is some debate over what’s happening in the mantle beneath the island, but there are no eruptions happening at the surface. Frazer and study co-author Jeffrey Park, a professor of Earth and planetary sciences at Yale University, used recordings from a seismic station on Bermuda of distant large earthquakes around the world to get an image of Earth down to about 31 miles (50 km) below Bermuda. They examined places where the seismic waves from these quakes suddenly changed. This revealed the unusually thick layer of rock, which is less dense than the other rock around it.