When the magnitude 9.0 Tōhoku earthquake struck off the coast of Japan in 2011, its seismic shivers did more than ripple through the planet.

At least one wave traveled 2,900 kilometers (1,800 miles) down to the boundary between Earth’s mantle and liquid outer core, where it was reflected right back to the surface.

And there, according to a new analysis of earthquake data from across Japan, it may have done something scientists have never identified before.