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PANGUITCH — Imagine a landslide involving a sheet of rock about 1 mile thick and larger than the entire state of Rhode Island traveling across the landscape at speeds up to 200 mph.

As mind-boggling as that sounds, scientists now believe it actually happened eons ago in southern Utah near present day Panguitch.

"Well, when we first figured it out, it blew our mind that this thing was as big as it is," consulting geologist Peter Rowley said.

Over the past several years, geologists hammered at rocks in the area, mapped the confusing geologic layers and pondered an enormous event that seems to have scrambled the landscape about 21 million years ago. Their extraordinary conclusion has been peer-reviewed by other experts.

"We think it is the largest landslide, on land, known in the world — ever," said Bob Biek, senior geologist for the Utah Geological Survey. "Right here in our own backyard in Utah."

It happened a long, long time ago, in a landscape far, far different from today. But almost everything visible today in the area stretching north from Panguitch Lake was part of the gigantic slide 21 million years ago.

An entirely different landslide made news recently when University of Utah scientists announced evidence of a big rockfall several thousand years ago at Zion Canyon. That slide is dwarfed, though, by the ancient slide near Panguitch which is much older and vastly larger.

Since they first announced their findings, scientists have learned that the landslide near Panguitch was even larger than they originally thought. They've also found surprising new evidence that such gigantic slides happened more than once.

Utahns are advised to not lose any sleep over the dangers, though. The chances of it happening again in this region are minuscule. But the story of the big slide is one of Utah's most jaw-dropping science yarns, and it may be a lesson for areas that are more volcanically active.

The first clues of a possible ancient landslide in the Panguitch area were observed by researchers in the 1970s, but the astounding scale of the event was not understood at the time.

"It's inconceivably big," Biek said.

The clues are so subtle that geologists overlooked most of them for decades. A renewed study of the area starting about 10 years ago began turning up evidence that eventually brought the amazing big picture into focus.

On a hilltop near Panguitch, there are boulders that were crushed or fractured by some incredible force.

"All this stuff we're standing on was just squished," Rowley said as he pointed out a highly fractured boulder.

On a slope overlooking Panguitch Lake, he inspected an area that appears to have been deformed by some violent cataclysm.

"It's a confused, chaotic appearance," Rowley said. "There are little shears going through, and the whole thing is just a big mess."

Imagine a landslide involving a sheet of rock about 1 mile thick and larger than the state of Rhode Island traveling across the landscape at speeds up to 200 mph. As mind-boggling as that sounds, scientists now believe it actually happened in Utah. (KSL TV)

A few miles away, Biek pointed to a layer of rock that was apparently superheated so fiercely by the friction of moving rock that it melted and later cooled into glass.

"Literally glass," Biek said. "It looks like obsidian."

As scientists put such clues together, he said, it added up to an event that is stunning for its size and velocity.

"This landslide, which is 60 miles long from north to south and is 40 miles long from east to west, moved more or less as a single sheet," Biek said, "very rapidly."

In fact, the gigantic mass of rock and dirt apparently slid about 20 miles to the south in a matter of minutes.

"If you were driving from Beaver to Cedar City on Interstate-15 with its 80 mile-an-hour speed limit, this thing would have rolled right over the top of you," Biek said. "We think it was well in excess of 100 miles an hour."

Of course, there was no I-15 then; there were no people anywhere that long ago. But animals in the area would have seen their world turned upside-down and inside out.

"It would have been a very bad morning or afternoon, back in the early Miocene," Biek said, referring to an ancient geologic epoch.

In the latest mind-boggling twist to the investigation, geologists have found evidence of another gigantic slide just north of Bryce Canyon National Park. The current studies suggest the second slide was smaller, but still among the champion landslides of all time.

Why did all this happen? It appears that several large volcanoes were actively laying down layer after layer of lava and mudflows on top of an unstable, greasy layer of clay.

"Finally, as you piled higher and higher, the whole mass slid," Rowley said.

Could it happen again?

Rowley believes the scenario is extremely unlikely to repeat itself in the Utah of today, which has no active volcanoes. But in other areas with huge volcanoes such as the Pacific Northwest, it's a possibility.

Someday, there could be a very bad morning in the Anthropocene, the epoch of humans.

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