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Great Salt Lake, in northern Utah, is so named for three reasons: it’s big, it’s wet, and it’s full of salt. But usually, that last property isn’t quite so obvious as it is right now – because since Fall of 2019, the Great Salt Lake has been increasingly dotted with large, white mounds of a substance known as mirabilite, or Glauber’s salt.

It’s a phenomenon local state park rangers have never seen before – and for good reason: before 2019, mirabilite mounds weren’t known to turn up at the Lake. In fact, they’re rather rare on Earth, and have only been found at a few locations, mostly in the Arctic and Antarctic.

We say “on Earth,” because mirabilite has a bit of a cosmic reputation. The conditions needed for it to collect in mounds like this are quite particular: it needs water fed from warm, sodium-sulfate-rich springs; a cold, dry environment; a lake level below 1,300 meters (4,194 feet) – features that remind some (arguably optimistic) researchers of the conditions on Mars.

“[The Mars connection] is more of a hypothesis and is based on satellite imagery,” Elliot Jagniecki, a senior geologist with the Utah Geological Survey, told Gizmodo. “Images show that Mars does have topographic mounds that some researchers think could be similar in composition, but it’s still an unknown.”

But if mirabilite mounds are so strange and rare, why are they coming to Utah? According to Jagniecki, they’ve always been there – we just couldn’t see them until recently. “We’re thinking the mounds are a response to a few things, including low lake levels, the arid climate, and low water input,” he tod Gizmodo. 

“When lake levels are higher, the underground springs are normally covered in salt water,” he explained. “So, they’re usually not visible, but with the lower water levels, now we can see them form.”

The Great Salt Lake may be the largest saltwater lake in the Western Hemisphere, but it’s not a patch on what it used to be: about 14,500 years ago, the lake was called Lake Bonneville, and it was more than 10 times the size of today’s supposedly “great” reservoir.

But with the end of the last ice age, warming temperatures led to this Lake Michigan-sized body of water drying up over the years. It’s such an extreme process that it can almost be witnessed in real-time: since 1875, the Great Salt Lake has shrunk by more than 40 percent, and as water levels dip to unprecedented lows local scientists have raised concerns that the state is “on the doorstep of a catastrophe.”

So rising temperatures and lower water levels may spell disaster for the humans and animals that live around the lake, but in the short-term, it turns out it’s quite the tourist attraction. As mirabilite is brought from the subsurface of the lake to the air by salt-water springs, and the cold air crystallizes the minerals into mounds, park rangers are leading tours of the mirabilite mounds to the public.

“They’ve… gotten bigger, with one measuring [1 meter] 3 feet in height,” Great Salt Lake Park ranger Angelic Anderson told Gizmodo. “Last year there was also one that was [10 meters] 35-feet long.”

Sightseers to the lake will have to be careful: mirabilite mounds are very fragile, and walking on them or collecting samples could cause them to break down into a white powder called thenardite. By summer, when warmer temperatures stop the minerals from crystalizing, the mounds will be replaced by thenardite entirely.

But barring some unforeseen massive flood, they should come back again in a few months. And by the looks of things, there’ll be even more of them for future tourists to check out.

“In 2019, the first year we spotted them, there were about four formations,” Anderson said. “But this year, we’ve recorded 15, the most we’ve ever seen.”

Katie has a PhD in maths, specializing in the intersection of dynamical systems and number theory.

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