Researchers have discovered that the underside of the North American continent is dripping away in blobs of rock—and that the remnants of a tectonic plate sinking in Earth’s mantle may be the reason why.

A paper published in Nature Geoscience describes the phenomenon, which was discovered at The University of Texas at Austin. It’s the first time that “cratonic thinning” may be captured in action.

“We made the observation that there could be something beneath the craton,” said the study’s lead author Junlin Hua, who conducted the research during a postdoctoral fellowship at UT’s Jackson School of Geosciences. “Luckily, we also got the new idea about what drives this thinning.”