A geologist has found part of a lost ocean that existed long before the Atlantic

Earth isn't the steadfast planet we assume it to be. Its continent-size slabs constantly move, buckle, and vanish beneath each other over the millennia, all while hardly leaving a trace.

But geologist Roi Granot, a senior lecturer at Ben Gurion University in Israel, says he's discovered the most ancient slab of seafloor on Earth to date.

The roughly 60,000-square-mile piece of crust has been hiding below the eastern Mediterranean Sea for about 340 million years. That means it's from right around when Earth's landmasses came together to form the supercontinent Pangea, which later separated into the continents we recognize today.

It's about 70% older than any other seafloors researchers know of, including those of the Atlantic and Indian oceans.

What's more, Granot thinks the ancient slab might be a relic of Earth's long-lost Tethys Sea (or Ocean), which we've only found fragments of in the mountains of countries like Cyprus, Turkey, and Oman.

"[W]e don't have intact oceanic crust that old … It would mean that this ocean was formed while Pangea, the last supercontinent, was still in the making," Granot wrote in an email to Business Insider.

No one had spotted the slab before because it's buried under more than 8 miles of sediment, according to Granot's new study about the slab, published Monday in the journal Nature Geoscience.

To test his hunch that the Mediterranean Sea was hiding something big, Granot conducted four research cruises from October 2012 through October 2014.

A crew towed three large sensors behind a boat, zigzagging across the sea during each trip to hunt for magnetic anomalies - the signatures of magnetic rocks locked in crust that was made by undersea volcanic ridges - buried deep beneath miles of ocean sediment.

A pattern of magnetic anomalies that's different from any surrounding seafloor, Granot reasoned, might detect the existence of an ancient block of crust trapped inside much younger crust.

And after 2 years of gathering data, his results revealed just that.

"I was shocked," said Granot, who was stuck on a 16-hour flight when he finished processing the data. "The picture was quite clear - I see oceanic crust! Since I had no one to share my new understanding, I had to walk back and forth in the airplane until [we] landed."

The findings could mean the Tethys Ocean formed about 50 million years before scientists thought.

"But we are not sure that it is really part of the Tethys Ocean. It could be that this oceanic crust is not related at all," Granot said, noting that it instead may be part of some other, unknown ocean bottom.

And aside from rewriting textbooks on plate tectonics, Granot says the discovery "could also help to understand heat flow in the eastern Mediterranean, which in turn will help to assess the potential of hydrocarbon [oil and gas reserves] in that region."

Other geologists will likely be working to confirm this finding, and Granot noted in the paper that the different tectonic possibilities that may have generated this discovery should all be tested in future studies.