Groundwater discharge into Great Salt Lake was historically understood to be under 3%. But the actual number may be closer to 12% — and that may have something to do with the lake’s "mystery islands."
“The Great Salt Lake’s mystery islands, I think that’s now the famous name, that’s what it’s been called in the press," said Ebenezer Adomako-Mensah, a grad student and researcher in Bill Johnson’s lab at the University of Utah.
First reported in July 2025, the mystery islands he’s referring to are small, circular, freshwater-fed reed-covered mounds found in Farmington Bay and other parts of Great Salt Lake.
“So in the north, there’s a lot of circular features closer to Fremont Island, and then Howard Slough,” he said.
The islands were first noticed in satellite imagery in 2023 and later confirmed to be forming around a freshwater nucleus. But Adomako-Mensah said that more important than that water’s presence is its rate of discharge, which doesn’t seem to be very high.
“Though we have huge amounts of freshwater" underneath Great Salt Lake, Adomako-Mensah said, "this groundwater is not moving." And that groundwater, the same water at the center of these mystery islands, is also extremely old.
“It’s been there for thousands of years, like 20,000, 40,000 years,” Adomako-Mensah said.
The age and immobility of that water suggest a slow, constrained system, one that has likely influenced sediment and plant growth over long time scales. But it also raises concerns about how the resource could be used. Especially as Great Salt Lake continues to shrink.
“We know that, okay, this is a resource, but then it still needs further characterization to see how sustainable it is,” Adomako-Mensah said.
For now, the mystery islands don’t point to an easy fix for Great Salt Lake. Instead, they reveal a hidden connection to a water system that exists beneath the lake, but one that moves slowly, replenishes over millennia, and demands caution before action.
