Antarctica’s McMurdo Dry Valleys are one of the coldest, most inhospitable places on Earth—and are the closest terrestrial analog to the harsh martian desert. For decades, scientists have thought that beneath a thin permafrost layer, the valleys were ice-cemented earth. But new data suggest that there are zones of liquid water hundreds of meters below the surface. A deep, briny “subpermafrost” groundwater network could harbor a hidden ecosystem, offering tantalizing clues to a possible martian habitat.

The Dry Valleys are a line of nearly snow- and ice-free valleys in the West Antarctic Ice Sheet at the edge of the Ross Ice Shelf. The region is dotted with scattered lakes and filled with salty soils and frozen permafrost. In warmer months, meltwater from glaciers and melting permafrost create a shallow, very briny groundwater system that could provide a kind of oasis for life in the frozen desert.

Some researchers have hypothesized a much deeper, salty groundwater system beneath the Dry Valleys. One possible surface sign of such a deep system is Blood Falls, a dark red oozy slush of concentrated, iron-rich brine that seeps out at one end of the Dry Valleys, where Taylor Glacier meets Lake Bonney. The Blood Falls ooze also contains a highly active microbial community. Data from a 1970s international research effort to explore the subsurface of the region, called the Dry Valleys Drilling Project, found tantalizing geophysical evidence—seismic and electrical resistivity data—that hinted at liquid water deep below. But boreholes also drilled as part of that project found only frozen earth.

To take another look at what might lie beneath, Jill Mikucki, a microbial ecologist at the University of Tennessee, Knoxville, and her colleagues partnered with SkyTEM, a Denmark-based airborne geophysical survey company. The researchers had a helicopter fly a giant transmitter loop over the landscape, inducing an electrical current in the ground, and measured the resistance to the current as far as 350 meters below the surface (see video below). The sensor is ideal for this work, Mikucki says, because it can distinguish highly conductive salty brines from highly resistant frozen water.

The researchers identified two distinct zones of low resistivity—indicating concentrated brines, or extremely salty ground water—which appear to create subsurface links between glaciers, lakes, and possibly even McMurdo Sound, the team reports today in Nature Communications.

Lakes that look isolated when seen from the surface may actually be connected hundreds of meters below the permafrost—and, if Blood Falls is typical of what that briny water holds, there may also be a vast ecosystem lurking down there, she says. And that suggests that the Dry Valleys may offer an entirely new terrestrial analog to help in the hunt for life on Mars, Mikucki adds. “On Mars, the subsurface is the place to look. It’s less harsh, and could be where life could have found relief.”

The finding also hints that such deep ground water may hold a previously hidden influence over the geochemistry and biological productivity of McMurdo Sound and the surrounding waters. The data suggest that some of the ground waters seep into McMurdo Sound, Mikucki notes. If Blood Falls does indeed reflect their geochemical makeup, then they may carry abundant nutrients—particularly iron and silica—into the iron-poor Southern Ocean.

The finding also sheds new light on the geological history of the region, says John Priscu, a polar ecologist at Montana State University, Bozeman. Decades of data collected as part of the McMurdo Dry Valleys Long Term Ecological Research program have turned up some puzzles, such as surprising chemical gradients in lake waters that didn’t quite make sense. Deep brines would add a new layer to the story, he says. But first, he adds, “the next step will be to verify the existence of these brines via direct sampling.”

The find is “exciting and raises some interesting questions,” says Joseph Levy, a planetary geologist at the University of Texas, Austin. But he urges some caution in interpreting the low-resistivity anomaly as a river of liquid brine, noting that “it’s an art to interpret” what combination of salt, water, sand, and other species will result in a particular conductivity.

Furthermore, it’s not clear exactly how this finding might help scientists understand Mars. Whether there is a deep groundwater system on Mars “is a much harder question to answer,” says Levy, who has studied both the Dry Valleys and the martian surface, hunting for analogs. “People have been looking for aquifers in the upper hundred meters of Mars with radar,” he says—so far, without success.

But, Levy says, in recent years there has been an increasing appreciation for salts and how they might create intragranular films of water: Instead of the deep briny lakes or aquifers one might find on Earth, scientists are now looking to “small pockets of briny soils that resist freezing and are chockablock with nutrients.” This finding, he adds, raises the possibility that we’re not looking for subsurface brines [on Mars] in the right way.”