Researchers identified a leak in Earth's oceans fifty years ago.

50 years ago, researchers discovered a leak in Earth’s oceans

                   

A photo of a remotely operated vehicle exploring the Mariana Trench. It appears as a cube hovering over a lighter blue patch surrounded by darkness.

A remotely operated vehicle in the western Pacific explores the Mariana Trench, the deepest oceanic trench in the world. Earth’s surface and mantle exchange water through such trenches, but the mantle may be taking on more water than it’s giving back, resulting in the ocean slowly shrinking over millions to billions of years.

The oceans of the world may be gradually shrinking, leaking slowly away into the Earth’s mantle…. Although the oceans are constantly being slowly augmented by water carried up from Earth’s interior by volcanic activity … some process such as sea-floor spreading seems to be letting the water seep away more rapidly than it is replaced.

Update

Scientists traced the ocean’s leak to subduction zones, areas where tectonic plates collide and the heavier of the two sinks into the mantle. It’s still unclear how much water has cycled between the deep ocean and mantle through the ages. A 2019 analysis suggests that sea levels have dropped by an average of up to 130 meters over the last 230 million years, in part due to Pangea’s breakup creating new subduction zones. Meanwhile, molten rock that bubbles up from the mantle as continents drift apart may “rain” water back into the ocean, scientists reported in 2022. But since Earth’s mantle can hold more water as it cools (SN: 6/13/14), the oceans’ mass might shrink by 20 percent every billion years.