Reeds do not grow on salt flats.

Yet dozens of near-perfect circles of them have shoved their way up through the crust of Farmington Bay, in the south-eastern corner of Utah’s Great Salt Lake. Some circles run 100 metres across. Stalks stand about five metres tall. Reporters at The Salt Lake Tribune have called them phragmites oases. Others went with mystery islands. University of Utah geoscientists have spent years drilling into them to find out what is doing the shoving, and the answer is fresh water, pushing up from below under pressure, in quantities nobody had budgeted for.

What the helicopter picked up

In February 2025, a survey crew flew a helicopter over Farmington Bay and the northern tip of Antelope Island with an electromagnetic sensor dangling underneath, tracing 154 miles of flight lines. Airborne electromagnetic surveying fires a changing magnetic field into the ground and listens for the echo. Brine conducts electricity far better than fresh water does, so telling the two apart comes down to how much the ground resists a current.

Michael Zhdanov and colleagues at the university’s Consortium for Electromagnetic Modeling and Inversion processed the returns and published them in Scientific Reports in late February. Near the surface, brine. Around ten metres down, the signal flips to fresh, and it keeps reading fresh right across the survey area.

Airborne gear only sees about 100 metres down, though. To get at the bottom of the thing, Zhdanov’s team inverted magnetic data to map the hard basement rock under the basin, and the shape of it was odd. Beneath most of the playa the basement sits shallow, less than 200 metres. Then it falls off a cliff to between three and four kilometres, with the drop-off directly under one of the reed mounds. Everything in that gap is sediment, and the university’s analysis puts fresh water in its pore space all the way down.

One survey, one small slice of a very large lake.

Zhdanov calls the work so far a pilot.

Water older than the lake it hides under

Graduate student Ebenezer Adomako-Mensah spent two years riding an airboat or a mountain bike out onto the drying mudflats, checking pressure gauges sunk into the reed mounds. His supervisor, hydrologist Bill Johnson, had the wells bored and the piezometers installed in the first place, hunting the same water from ground level that Zhdanov’s team had just mapped from the air.

Their results, published in the Journal of Hydrology, describe a previously unknown boundary nine metres below the playa, the saltwater lens, where fresh water meets brine. The reed circles are the places where pressurised fresh water finds a gap and pipes straight through. Adomako-Mensah found that the bigger the circle, the fresher the water at its centre, turning saline again towards the edges.

Isotope work suggests the deep water has sat there for thousands of years. Johnson suspects some is left over from Lake Bonneville, the enormous freshwater lake that covered much of north-western Utah during the last ice age and left the Great Salt Lake as its remainder. That carries an unwelcome implication, as Johnson has noted: water that old moves too slowly to be topping up the lake anytime soon.

The south shore refuses to cooperate

Drive around to the other side and the tidy picture falls apart. Geophysicist Mike Thorne’s group ran electrical resistivity tomography at 16 locations along the southern shore, laying out electrode lines hundreds of metres long. Writing in Geosciences, lead author Mason Jacketta reported thick brine sitting just a few metres down at some sites and fresh water at similar depths at others, sometimes within walking distance of each other.

At Saltair the team found brine trapped beneath a buried layer of mirabilite, a sodium sulfate better known as Glauber’s salt. As Thorne told Eos, the flat monotony above ground hides a lot of lateral variation below it.

Why anyone is paying for this

How does a lake go from holding this much fresh water underneath it to being one of the saltiest bodies of water on the planet on top? Evaporation, mostly, and no way out. Great Salt Lake has no outlet, so the salt carried in by its rivers never leaves, only concentrates as water evaporates away. The U.S. Geological Survey puts its salinity anywhere from six to twenty-seven per cent depending on the section of lake and the season, two to seven times saltier than the ocean.

Great Salt Lake has also shrunk by roughly 70 per cent since 1989, thanks to upstream diversions and a drying climate, exposing around 800 square miles of lakebed. That sediment is now a dust problem, and the dust carries toxic metals into Salt Lake City and the towns along the Wasatch Front. Utah’s Department of Natural Resources and the Great Salt Lake Commissioner’s Office are funding the groundwater work for practical reasons.

Johnson has proposed drilling wells at the worst dust hotspots and flooding them, wetting the crust so it stops lifting into the air. Refilling the lake was never the pitch, and the aquifer could not manage it anyway.

Whether that is safe is the open question. Pull too much out and the upward pressure holding fresh water above the brine weakens, and the reed circles that revealed the system stop existing.

The number nobody has yet

Volume is still guesswork. Zhdanov’s method can estimate it given depth, width and pore space, but the survey covered a sliver of a 1,500-square-mile lake. Johnson puts it more bluntly: the volume looks large, and the flux, meaning how fast the water moves and how much can be drawn without breaking anything, is unknown.

One university and one state agency have sketched the outline of a resource that could rewrite water planning across the Wasatch Front, and an outline is all it is so far.