ON APRIL 1, 1994, PAUL Butcher, then the director of Colorado Springs parks department, received a chilling phone call from a frantic staff member. She told him that Balanced Rock—a 290-million-year-old red sandstone boulder naturally perched on a sloped ledge in Garden of the Gods Park—had fallen. Butcher panicked, his thoughts roiling with how disappointed and outraged both locals and visitors would be with the loss of the beloved, iconic landmark. He imagined the 700-ton boulder rolling downhill, with nothing to stop its tumble onto the nearby U.S. Highway 24, like a monstrously dense tumbleweed. Then he remembered the calendar, and realized it was a prank. “I never laughed,” Butcher, who is now retired, told Out There Colorado. “It’s not a great joke.”
In a way, the mere existence of Balanced Rock also seems like a prank, either geological or cosmic. The enormous boulder looks like it had been photoshopped onto the landscape, or photographed mid-roll, or carefully placed by aliens. But it’s no hoax and there’s no sorcery to it. Rather it is a prime example of a whole category of geologic formations called “precariously balanced rocks”—PBRs, for short. They’re exactly what you might expect. “It’s a rock balanced on top of another rock,” says Mark Stirling, who studies PBRs at the University of Otago in New Zealand. And if you think Colorado Springs’ landmark ought to have a more imaginative name, see also: Balanced Rock in Grand Junction, Balanced Rock in Rocky Mountain National Park, and Balanced Rock at the Rampart Range. And that’s just Colorado.
PBRs are more than just unusual geologic features—they’re a source of valuable scientific insight. They’re what are called “reverse seismometers” because their mere existence makes it possible to measure earthquakes that didn’t happen. If they’re still balanced, then the earth hasn’t moved enough to knock them over, at least in the last few thousand years, according to geologists David E. Haddad and J. Ramón Arrowsmith in their seminal 2011 report Geologic and Geomorphic Characterization of Precariously Balanced Rocks. So scientists study them to understand a region’s seismic history and, subsequently, predict what might come in the future. “They’re nature’s hilarious accidents,” says Amir Allam, a geologist at the University of Utah.