Geologists have discovered what they believe to be an Arctic Ocean-size reservoir of water lying deep beneath the Earth’s surface. The so-called “Beijing anomaly” could have a significant impact on the study of water as a catalyst for geological activity.

Michael Wysession of Washington University in St. Louis and Jesse Lawrence of UC San Diego discovered the anomaly while creating a 3D model of the Earth’s mantle—the 1800-mi.-thick layer of rock just beneath the crust. To create the model, the pair analyzed both the speed and attenuation of more than 80,000 earthquake-induced seismic waves. Speed is an indicator of temperature, while attenuation, or the speed at which a wave decreases in strength, indicates composition.

The model revealed an area of very weak rock—where wave strength decreased quickly—about 620 mi. below the eastern border of the continent of Asia. Wysession says theoretical work suggests that saturation of the rock with water is the most likely source of the anomaly. “Wave attenuation is very susceptible to water,” Wysession says. “It dampens the energy of the wave.”

The geological conditions along eastern Asia are ripe to produce such a body of water, he says. As the Pacific plate slides below that of Asia, the rocks on the ocean floor, which can hold up to 15 percent of their mass as water bonded to mineral, are pulled into the mantle. “We used to think that water dehydrated at about 100 km [62 mi,] below the surface,” Wysession says. “But if the ocean floor is cold enough, and moving quickly enough, the water can enter the mantle.” The rocks transform and finally release the water when extreme pressure combines with temperatures between 1800 and 2700 degrees Fahrenheit.

The body of water beneath eastern Asia is the only large example of this phenomenon that the team identified from its model. But Wysession believes the discovery could have a profound impact on future geological studies. Geological “hot spots,” like Yellowstone National Park, he suggests, might actually be caused by smaller bodies of water in the mantle that weaken the rock and allow heat to escape.

“I think we are on the verge of changing our theories about what causes variations in our Earth,” Wysession says, “from being entirely due to temperature variation, to being due to the presence of water.”  —Emily Masamitsu