Scientists have discovered the “fossilized fingerprint” of a chunk of seafloor that was hiding beneath the Pacific Ocean in Earth’s mantle.

A new study shows that this fingerprint corresponds to a slab of Earth’s crust that began sinking into the mantle approximately 250 million years ago, at the dawn of the age of dinosaurs (252 million to 66 million years ago). This slab once formed part of the seafloor in the southeastern Pacific and could help explain a strange gap in the lowermost sections of the mantle — the middle layer of Earth’s crust that wraps around the planet’s core. “It’s giving us a glimpse into Earth’s past that we’ve never had before,” study lead author Jingchuan Wang, a seismologist and postdoctoral associate at the University of Maryland, said in a statement.

The sunken slab now sits sandwiched between the upper and lower mantle, in the mantle transition zone. This transition zone stretches between 255 and 410 miles deep beneath Earth’s surface, although it can expand and contract depending on heat currents circulating in the mantle, according to the statement.

Wang and his colleagues found the slab while exploring the mantle beneath the East Pacific Rise, a fast-spreading mid-ocean ridge located 2,000 miles off the coast of South America. The researchers used seismic waves to examine the types of rock beneath the seafloor and create a digital cross-section of Earth’s crust and mantle. The team published its findings in the journal Science Advances.

The cross-section revealed an unusually thick mantle transition zone beneath a portion of the East Pacific Rise roughly 220 miles east of Rapa Nui, which is also known as Easter Island. “This thickened area is like a fossilized fingerprint of an ancient piece of seafloor that subducted into the Earth,” Wang said. Subduction occurs when two tectonic plates collide and one dives beneath the other. Material from the subducting plate usually disintegrates in the mantle, where blazing temperatures recycle the rocks into magma. Remarkably, the newly discovered slab somehow escaped that fate.

“Usually, oceanic slabs of material are consumed by the Earth completely,” Wang said. The slab’s position indicates it traveled through the mantle at about half the speed researchers would normally expect for a subducting plate, according to the statement. This in turn suggests the mantle transition zone can act as a viscous barrier and slow the movement of sinking material, Wang said.