A long-lost tectonic plate that once underpinned what is today the South China Sea has been rediscovered 20 million years after disappearing.

The plate is known only from a few rock fragments from the mountains of Borneo and the ghostly remnants of its huge slab detected deep in Earth’s mantle. It was once a quarter of the size of the Pacific Ocean. Scientists have dubbed it the “Pontus plate” because at the time of its existence, it sat under an ocean known as the Pontus Ocean.

“It’s surprising to find remnants of a plate that we just didn’t know about at all,” Suzanna van de Lagemaat, a doctoral candidate at Utrecht University in the Netherlands, told Live Science.

Van de Lagemaat and her colleagues were initially studying the Pacific plate under the Pacific Ocean. Tectonic plates constantly move against one another, and the crust in oceanic plates is more dense than continental plates, so oceanic plates get pushed under continental plates in a process called subduction and disappear. Sometimes, however, rocks from a lost plate get incorporated into mountain-building events. These remnants can point to the location and formation of ancient plates.

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