The area around the Philippine Sea is better known for being a political hotspot than a geological one. But it turns out that the tectonic plate named for it is just as volatile. It’s located at the intersection of the Eurasian, Pacific and Indo-Australian plates — three plates, that, millions of years ago, swallowed up an ancient sea.

That’s the conclusion of a new study published in the Journal of Geophysical Research that sheds light on something scientists once thought they’d never be able to explore: the remnants of ancient tectonic plates. Using new technology that lets them peer into the past buried deep in the Earth’s mantle, scientists from National Taiwan University have concluded that 500 to 1,300 kilometers below the Earth’s crust lie plates that once sat at the bottom of what they call the East Asian Sea. They think that the long-lost ocean once sat between what are now the Pacific and Indo-Australian oceans, covering more than 15 million square kilometers of Earth’s surface about 52 million years ago.

In the grand scheme of geology, plate tectonics isn’t that old: Scientists only confirmed the theory that Earth’s crust — its outermost layer — is made of a bunch of plates around half a century ago. But new tools are already adding to geologists’ original concept of a surface filled with plates.

It turns out that even when plates are subducted, or sucked under other plates, they float within the lower layer, called the mantle, like a leaf sinking to the bottom of a swimming pool. It takes up to 300 million years for each plate to disappear into Earth’s core — and while it’s within the mantle, it can still be seen using seismic tomography, a tool that uses the seismic waves generated by earthquakes to peer inside our planet.

Geologist Jonny Wu and his team took that technology one step further by using software to map, “unfold” and stitch together 28 slabs, each of which sits at a different place within the Earth’s mantle. (Like that sinking leaf, the slabs that have been inside the mantle longer sink lower than those that have been there for less time.)

“Each of these plates starts to become part of a really intricate jigsaw puzzle with different sizes and shapes and times they must have existed on Earth,” says Wu — a 4-D puzzle that, once complete, wound up filling in a 52-million-year-old gap.

The researchers weren't looking for a lost sea. Wu and his colleague, John Suppe, were both at National Taiwan University when they became interested in the plate tectonics of the region. Since Southeast Asia is a notorious plate graveyard, says Wu, they decided to home in on the Philippine plate to find out more about the tectonics beneath Taiwan. “We thought, if we could deduce how big that plate was we could probably understand where it came from,” he explained.

That’s not what happened, though. After mapping one plate, they needed to do another, then another. “We were seeing things that we didn’t even realize existed,” says Wu. “We didn’t know whether to believe it or not until we put it all together.”

It took five years for Wu and his colleagues to gather enough evidence to support the existence of the sea, which they think shrank over time as more and more plates in the area were subducted and lost. The shape of the sea fits in with a literal gap in plate tectonics — an area in the vicinity of the theorized sea whose lithology geologists haven’t been able to figure out. “Twenty-eight slabs later, we felt like we could answer that question,” he laughs.

Now Wu and Suppe have joined the faculty of the University of Houston, where they will try to delve even further into the past that’s buried beneath the Earth’s crust. Although they’re still refining their method, which is only as good as the seismic data collected by monitoring stations throughout the world, they hope that their work is just the beginning of a new way of looking at Earth’s history from the inside out.

“We’re filling in the other half of plate tectonics by looking inside the interior of the Earth,” says Wu. “Who knows how many other oceans there are to be found?”

Erin Blakemore (@heroinebook) is a freelance journalist from Boulder, Colo. She is the author of "The Heroine’s Bookshelf" (Harper). 

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