A field of loose rubble under the summit of Mount Timpanogos hides enough frozen water to fill 600 Olympic swimming pools.

Nothing on the surface gives it away.

That figure comes from University of Utah geologists who spent the autumn of 2024 climbing to a cirque, a bowl gouged into the mountainside above Emerald Lake, to survey what lay beneath the rubble there. The hike alone runs roughly eight kilometres and climbs about 1,100 metres.

Their results, published in the Journal of Geophysical Research: Earth Surface, put the buried ice at 30 to 45 metres thick (100 to 150 feet) down the centre of the landform, with a total volume near 1.55 million cubic metres. Lead author Bronson Cvijanovich has compared that to the largest pyramid at Giza.

What sits up there is a rock glacier: ice under a carapace of loose rock several metres deep, creeping downhill at 8 to 20 centimetres a year.

How to weigh ice you cannot see

Ground-penetrating radar, the standard tool for sizing up a glacier, fails on these things. Rock mixed through and over the ice scatters the radio waves and the image degrades into noise.

So the team measured gravity instead.

Rock is denser than ice, which means the pull underfoot is fractionally weaker wherever the ice runs thicker. Cvijanovich made six trips up the mountain with a gravimeter, taking readings at 232 points on a grid spaced roughly 25 metres apart. The full data set appears in a survey published by the Utah Geological Survey.

The signal is so faint that the positions of the sun and the moon had to be accounted for, a detail picked up in coverage by Nautilus. A statistical model then weighed every plausible shape of buried ice against the readings and kept the most probable one, turning 232 numbers into a three-dimensional picture.

What is under the rocks

“Timpanogos Rock Glacier is surprisingly ice-rich. It is 83% ice and 17% loose rock,” said Cvijanovich, in an announcement from the University of Utah. Average thickness across the ice core came out at 18.8 metres, well short of the 150-foot maximum found along the centreline.

Glaciology professor Leif Anderson made the same point for anyone who has crossed a talus slope without a second thought, telling ABC4 that hikers rarely guess there’s another 120 feet of ice sitting beneath them.

Where the ice comes from

So how does a body of ice survive under a rock pile in one of the driest states in America? A companion paper in Geophysical Research Letters, led by Isaiah Davies, models the mechanism.

Snow banks up in the cirque, rock spalls off the steep headwall above and buries it, and buried snow melts far slower than bare snow. In a favourable year (heavy snowfall, a cool summer, a generous rockfall) the pile gains mass in pulses. Over a few thousand years, those pulses compact into glacier ice.

That paper also shifts the origin story. These are not ice age relics left over from a peak 21,000 to 18,000 years ago, but younger water stores assembled long after the big glaciers had gone.

The reservoir question

Utah has 836 known rock glaciers, tallied from satellite imagery and documented by Utah News Dispatch. Only one of them has actually been weighed.

In arid country, alpine ice serves as a hedge against drought, releasing water into streams once the regular snowpack runs out.

Using the area-to-volume relationship measured at Timpanogos alongside published data from ten other rock glaciers, the researchers scaled the estimate outwards: roughly one gigaton of water in Utah, close to 12 gigatons across the western United States, and about 48 gigatons in the world’s intact rock glaciers.

Those global figures rest on one landform imaged in detail and fifty thousand others outlined from space and multiplied. Cvijanovich has since described the rock cover itself as protective camouflage, calling these sites climate-resilient water stores that may still be adding ice, per Newswise.

There is a risk that comes with the reward. The original paper notes that a rock glacier becomes a hazard to the people living below it once it destabilises. KSL reported that this one also carries a potential flood risk, even as its slow-melting ice benefits Utah’s dry climate.

Anderson’s team found that in some years the whole rock glacier is bare rubble, while in other years (rarer now than they once were) rock-covered snow survives into September.

The ice is still gaining. It has simply become choosier about which years it will bother.