The Pacific and Atlantic oceans are the oldest on Earth, and the oldest large-scale oceanic crusts at their bottoms date back up to about 200 million years ago. But according to a new paper, the ocean crust of the eastern Mediterranean Sea could be much older at about 340 million years old.

In a study published Monday in the journal Nature Geoscience, Roi Granot from the Department of Geology and Environmental Sciences at Ben-Gurion University of the Negev in Israel identified the crust under the Herodotus Basin in the eastern Mediterranean as the oldest oceanic crust still in place. Granot used magnetic data from the region to analyze the nature of the crust, which lies covered under 6-10 miles of sediment.

Ocean crust is formed when magma from Earth’s mantle erupts, usually at mid-ocean ridges, and is cooled by the surrounding water. It is also typically thinner than continental crust. The minerals in the newly-formed rocks get magnetized and align with the direction of the planet’s magnetic field. This makes ocean crust a good time record of changes in the orientation of Earth’s magnetic field.

A satellite image of the Mediterranean Sea. NASA

The rocks that form the crust in the Herodotus Basin were “characterized by magnetic stripes” that allowed Granot to place their age at as much as 340 million years.

In a statement, Granot said: “The area is covered by thick sedimentary coverage, making it unclear precisely how old the crust is and whether it is even oceanic at all. With the new geophysical data, we could make a big step forward in our geological understanding of the area.”

According to Granot, the crust could be a leftover from Tethys Ocean, an ancient body of water that existed before the Atlantic and Indian oceans. If true, the new finding could push back our understanding of when the ocean formed, which current estimates put at about 270 million years ago.