ZION NATIONAL PARK, Utah — The same unique geological quirks that lure people to the majestic slot canyons of the desert Southwest also make them deadly.

The curved, narrow sandstone walls glow in the sun with a cathedral-like light. When it rains, however, they can fill with raging rain water in an instant, leaving people with no escape.

That’s exactly what happened Monday evening to a group of seven hikers from California and Nevada who were trapped by floodwaters in the popular Keyhole Canyon in Zion National Park. The canyon is as narrow as a window in some spots and several hundred feet deep.

Six have turned up dead. One is still missing.

“That little bit of rain can turn what was a very comfortable daylong excursion into a horror story, literally in a split second,” said Colorado-based canyoneering expert Steve Allen.

The flood marks one of the deadliest weather-related disasters at a national park in recent history, park service officials said. It evoked memories of a 1997 incident near Page, Arizona, where 11 hikers died after a wall of water from a rainstorm miles upstream thundered through Lower Antelope Canyon, a narrow, twisting series of corkscrew-curved walls located on Navajo land.

The deadly events at Zion happened at the same time flash floods tore through a small community on the Utah-Arizona border just south of the park, leaving at least 12 people dead who were in two cars swept downstream.

Outdoor enthusiasts are attracted to slot canyons by what Allen calls “eye candy” created by nature. Water flows through cracks in the earth and gradually erodes the sandstone underneath, leaving caverns narrow enough for hikers to touch the sides when they stretch out their arms.

Adventure-seekers also get a rush from the combination of rock climbing, swimming, hiking and cave exploration, all in a setting totally different from the surrounding desert, Allen said.

The Keyhole Canyon at Zion in southern Utah where the hikers were killed is what canyoneers call a “rap and swim” canyon, full of a series of drops where hikers rappel down into pools of water, Allen said. It’s considered an entry-level canyon for people who have some experience but are still new to the sport.

Zion spokeswoman Aly Baltrus said some members of the group were new to canyoneering, but they took a class to prepare.

Park rangers regularly warn hikers that flash flooding during monsoon season can turn southern Utah’s beautiful canyons into deadly channels of fast-moving water and debris. But dozens of adventure-seekers go anyway.

This group was already in the canyon when a flash flood warning led park officials to announce they were closing their canyons. By that time, park officials say, there was no way to alert them to the violent floodwaters coming their way.