In a discovery that challenges conventional geological understanding, researchers from The University of Manchester have found that gases originating deep within Earth’s mantle are seeping into groundwater—far from any active volcanic or tectonic regions.

The study, published in Nature Geoscience, revealed the presence of helium and argon isotopes in underground water supplies from 17 wells in the Palouse Basin Aquifer, a geologically stable region in the northwestern United States.

“Think of it like having a small puncture in your car tire,” said Dr. Rebecca Tyne, Dame Kathleen Ollerenshaw Fellow and lead author of the study. “We've discovered a steady trickle of gases coming from deep within Earth, even though there's no obvious volcanic activity on the surface.”

Traditionally, scientists believed that gases from Earth’s interior either remained trapped underground or were released primarily through volcanic activity. However, this research reveals a form of passive degassing—where gases migrate slowly through Earth's crust without volcanic assistance.

The gases identified in the groundwater, including helium-3 and argon-40, are inert and chemically non-reactive. Their presence indicates that they originated from a deep mantle source, specifically the sub-continental lithospheric mantle, a layer located many kilometers beneath Earth’s surface.

“We found evidence of mantle-derived gases in 13 out of the 17 wells,” said co-author Dr. Mike Broadley, NERC Independent Research Fellow at the University of Manchester. “These gases—especially helium-3 and argon-40—do not form in the atmosphere or in shallow rocks, they come from a layer of the mantle.”

The highest concentrations of these gases were detected in the oldest and deepest groundwater samples—some over 20,000 years old—suggesting that the gas migration has been ongoing for millennia. Additionally, the researchers noted a consistent pattern in the gas signatures, indicating they may have traveled together from a shared mantle source.

“This passive degassing of the mantle may be an important, yet previously unrecognized process,” said Dr. Tyne. “These findings will help our understanding of how our planet's interior works and how much gas is escaping into the atmosphere over time. It could even play an important role in the geologic carbon cycle.”

The team plans to extend their research to examine whether this phenomenon is occurring globally by analyzing groundwater in other stable regions around the world.