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.

Cratons are very old rocks that are part of Earth’s continents. They’re known for their stability and ability to persist for billions of years. But sometimes cratons undergo changes that can affect their stability or that remove entire rock layers.

For example, the North China Craton lost its deepest root layer millions of years ago.

What makes the discovery of cratonic dripping special, said the researchers, is that it’s happening right now. This allows scientists to observe the cratonic thinning process as it occurs.