A layer only a few to a few dozen kilometers thick may be draped across the boundary between Earth’s core and mantle, and researchers say it likely consists of ancient ocean floor pushed deep underground over geologic time. That is the picture emerging from a study led by The University of Alabama, published in Science Advances, which used seismic data from Antarctica to probe a vast stretch of the Southern Hemisphere nearly 2,000 miles below the surface. The team found evidence that ultralow velocity zones, or ULVZs, are not just isolated patches in a few places. Instead, they may be widespread along the core-mantle boundary. These zones slow seismic waves and appear denser than the surrounding deep mantle. The researchers argue that the best explanation is old oceanic material that sank through subduction, then spread and accumulated along the bottom of the mantle. “Seismic investigations, such as ours, provide the highest resolution imaging of the interior structure of our planet, and we are finding that this structure is vastly more complicated than once thought,” said …