The ocean floor is the ultimate recycling center. While the Earth’s continental crust can exist for billions of years, movement of tectonic plates causes subduction, which is when the ocean crust is shoved down into the molten mantle. So the ocean floor rarely lasts longer than 200 million years. But researchers in the Mediterranean Sea have found a chunk of ocean floor that may be 340 million years old, dating back to the creation of the supercontinent Pangaea, reports Dave Mosher at Business Insider.

The floor of the Mediterranean is not well studied since much of it is covered by miles-deep sediment, making exploring impossible. So instead of visiting the crust, researchers measured what they call magnetic anomalies—stripes of differing magnetic orientations recorded in the crust—to examine the sea floor. These magnetic stripes are created as the ocean crust forms along mid ocean ridges. As the magma cools, magnetic particles in the solidifying rock orient themselves with Earth’s magnetic field.

"Changes in the magnetic field's orientation over time are recorded in the ocean floors, creating a unique barcode that provides a time stamp for crust formation," says Roi Granot of the Ben-Gurion University of the Negev in a press release.

Granot and his team towed magnetic sensors to map 4,300 miles of the sea floor around the Herodotus and Levant Basins in the eastern Mediterranean basins between Turkey and Egypt. It took four research cruises between 2012 and 2014 to cover such a large area, reports Emily Benson at New Scientist.

As they mapped, they quickly realized that their sensors were picking up on magnetic stripes that indicated a previously undiscovered mid-ocean ridge. “Here I am in the middle of the eastern Mediterranean and I see this beautiful feature that crosses the entire sea, from north to south,” he tells Benson. “That feature can only be created by oceanic crust.”

But Granot didn't realize just how old that crust was until he finished processing the data on a 16-hour flight home, Mosher reports. He was so excited he had to walk up and down the plane till they landed. “I was shocked,” Granot says. “[W]e don't have intact oceanic crust that old … It would mean that this ocean was formed while Pangaea, the last supercontinent, was still in the making.”

Granot suggests the area of ancient crust may be part of the ancient Tethys Ocean, which would mean that body of water formed 50 million years earlier than previously thought. He also tells Mosher it may be from an unrelated ocean ridge.

Not everyone is convinced that Granot’s find is as old he claims. Uri ten Brink of the U.S. Geological Survey tells Benson that the heavy layer of sediment may make magnetic signals difficult to interpret correctly. He also says the survey area is small, which makes it difficult to identify magnetic mineral stripes. The only way to tell will be with more mapping.