Something that happens only in horrifying end-of-the-world movies just took place in the United States: the Earth opened up and "swallowed itself" in the foothills of Wyoming's Bighorn Mountains.

The hunting group SNS Outfitters and Guides was the first to alert the world about the geologic phenomenon by posting a picture of it on its Facebook page.

The stunning photo showed a portion of the Wyoming Bighorn Mountains that have eroded, creating a hole with seeming puzzle pieces of earth that do not fit in.

"This giant crack in the Earth appeared in the last two weeks on a ranch we hunt in the Bighorn Mountains," the group, known for offering guided elk, antelope, deer, moose and bear hunting expeditions, said on its Facebook page.

SNS Outfitters and Guides first called the crack on the mountain foothills as "The Gash."

"It is a really incredible sight," the group said.

It estimated the opening to be 750 yards long and 50 yards wide in spots. What made "The Gash" even more terrifying is that it goes many, many stories deep.

Other explorers have also gone and taken pictures of the crack, which they also posted on social media.

After interest about "The Gash" peaked on social media, the Wyoming State Geological Survey (WSGS) came out with an explanation about the geologic phenomenon.

According to the WSGS, the crack was a result of "the apparent active landslide" in the southern end of the Bighorn Mountains.

"The Crack... looks to be a relatively large yet slow-moving event. It appears that this event may be due to groundwater creating weakness in an unstable hillside," the organisation said, as quoted by CNN.

Dave Petley, a professor at the University of East Anglia in the United Kingdom, meanwhile, explained that ground water in the mountains "did play a role" in the appearance of the crack.

"In general, (water's) role is to change effective stress, not to provide lubrication. There is a good chance, though, that there is a progressive failure and that the changes in the behavior of the springs occurred as the internal damage of the slope (reconfigured) itself as the deformation developed," Petley wrote in his "Landslide Blog."