A perplexing amount of the planet's water slips deep into Earth's interior at some tectonic boundaries, a new comprehensive seismic study of the Mariana Trench region has revealed, reports Phys.org.

The first-of-its-kind study, which took place along the deepest trench in the world, found that the amount of water slipping through the "cracks" along subduction zones — places where tectonic plates plummet deep into the Earth — could be as much as four times what previous estimates had suggested. And the water is disappearing far deeper into the planet's mantle as well, as much as 20 miles beneath the seafloor.

"People knew that subduction zones could bring down water, but they didn't know how much water," said Chen Cai of Washington University, first author on the study.

"This research shows that subduction zones move far more water into Earth's deep interior — many miles below the surface — than previously thought," added Candace Major, a program director in the National Science Foundation's Division of Ocean Sciences, which funded the study.

Subduction zones, which cause most of the planet's deepest trenches, are capable of pulling water down into the Earth's interior due to the immense pressure and temperature conditions they generate. Rocks along these boundaries can act like sponges, absorbing the water and dragging it down with them. It wasn't until now that scientists knew just how much water got sucked down, and the numbers are so substantial that it's unclear where the water goes.

Where does all that water go?

The good news is, much of the water most likely bubbles back up at some point. For instance, volcanoes spew water vapor back into the atmosphere as part of this water cycle. The only problem is, measurements of water vapor released by volcanoes falls short of the estimates from this study of the amount of water lost through subduction. In other words, it would appear that the amount of water going into the earth greatly exceeds the amount of water coming out.

So at this point, where the water goes is a bit of a mystery. It's not being lost — Earth's sea levels haven't been sinking dramatically throughout the planet's history — but more research will need to be done to figure out how the water cycle sustains itself.

"The estimates of water coming back out through the volcanic arc are probably very uncertain," said Doug Wiens, research advisor on the study. "This study will probably cause some re-evaluation."

One possible explanation is that not all subduction zones are created equal. Perhaps the conditions along the Mariana Trench are more extreme than at other places around the planet, where less water is lost. That's a hypothesis for another study, however.

"Does the amount of water vary substantially from one subduction zone to another, based on the kind of faulting that you have when the plate bends?" asked Wiens. "There's been suggestions of that in Alaska and in Central America. But nobody has looked at the deeper structure yet like we were able to do in the Mariana Trench."

A huge amount of water is sinking through the planet's tectonic fault lines
A perplexing amount of the planet's water slips deep into Earth's interior at some tectonic boundaries, a new study has found. But where does it go?