City Scene: An Ocean Beneath Our Feet

China’s capital has no shortage of environmental woes: rivers are disappearing, land is sinking, and now the government is spending RMB 100 billion to build an aqueduct to transport water from the Yangtze River. Come spring’s sandstorms, the place starts to resemble the lost, and very parched and sandy city of Petra. But underneath it all, deep within the bedrock below, lies … an ocean?

Well, sort of. “We used the word ‘ocean’ to describe the approximate volume of how much water we think might be contained inside this attenuation anomaly,” explains Jesse Lawrence, a researcher from the University of California at San Diego who discovered the geological mystery dubbed the “Beijing anomaly.” It may sound like a baddie from a horror flick, but the anomaly is in fact an enormous layer of water-infused rock, deep beneath eastern Asia.

Rather than discovering water, Lawrence and his former professor, Michael Wysession, a researcher at Washington University in St. Louis, happened upon it via seismic measurements from around the planet, through a process Lawrence likens to listening to muffled music through a wall. They noticed that seismic waves traveling beneath Asia appear to slow down and dampen, or attenuate, in a big way. A distortion this huge, says Lawrence, can only mean the Big Dumpling is sitting on rocks full of water – an ocean’s worth of it.

But how did it ever get down there? And (wonders the wily entrepreneur) how on earth do we get at it? Lawrence explains that water seeps down from the earth’s surface to the earth’s depths through a quarter-billion-year process called subduction. Typically, water trapped in rock begins its return to the surface after sinking 200 kilometers, eventually to escape through volcanoes and other fissures. In this case, however, slabs of cooler, water-enriched minerals beneath us have continued downwards to depths of 600 to 1,200 km, taking about 13-20 million years to do so, Lawrence estimates.

Eventually, at such infernal depths, the rocks destabilize, releasing water that later reenters the earth’s surface water cycle. Of the impact on Beijing, says Lawrence, “the anomaly is deeper than the deepest earthquakes, so no fear there.” But the depths are so great that the chance of squeezing water out of them is as tiny as the percentage of water trapped in the rocks – about 0.1 % by weight. “Even if we could reach these depths,” says Lawrence, “it would be nearly impossible to extract the H2O from the minerals. Desert sands likely have as much or more water.” Nonetheless, given the capital’s increasing lack of liquid, perhaps it’s time to start digging. Alex Pasternack