They walked 225 yards away from the boardwalk and into an isolated area, officials said, only a thin, feeble crust of earth separating their feet from the acidic water bubbling beneath.

Then, somehow, Colin Scott slipped, and as his sister watched, the 23-year-old tumbled into one of the boiling springs. Despite her immediate call for help and the prolonged search efforts by park staff, her brother was never seen again.

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His flip-flops, reported NBC News, were one of the few things rescuers retrieved.

They halted their recovery mission Wednesday, park spokeswoman Charissa Reid told the Associated Press, “due to the extreme nature and futility of it all.” The water was highly acidic, Reid said, leaving “no remains left to recover.”

Yellowstone’s awe-inspiring hot springs have claimed 22 lives since 1890, park officials told the AP, but Scott’s was the first thermal-related death in 16 years. More than bear maulings or cliff falls, being burnt alive in a hot spring is perhaps the most horrifying way to die in Yellowstone.

For those who aren’t killed immediately, death can come slowly — hours, days or even weeks after the initial burns.

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“It’s devastating,” park ranger Jessica Korhut told NBC Montana, “not only to the families that are involved but also the folks that have to go in and rescue them.”

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Yellowstone has more than 10,000 thermal features, including geysers, hot springs, mud pots and steam vents, according to the park’s website. The hottest exist in the Norris Geyser Basin, where Scott died and where most of the features bubble well over the boiling point of 199 degrees Fahrenheit. (The elevation slightly reduces the boiling point.) It was there that the record-high temperature for any geothermal area in Yellowstone was recorded inside a scientific drill hole, at a piping 459 degrees Fahrenheit.

Signs litter such areas throughout the park, warning visitors to remain on boardwalks and keep pets away. Yet there’s something about the technicolor springs that lures otherwise rational observers toward their magnetic depths. They’ve killed or injured more people in Yellowstone than any other natural feature.

In his book “Death in Yellowstone: Accidents and Foolhardiness in the First National Park,” historian Lee H. Whittlesey dedicates the first chapter to thermal deaths and quotes an unnamed author, who, when contemplating the hot springs, once wrote that “death lurks in the path of those who venture near, fascinated by the dazzling array of hues.”

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Whittlesey writes that the first three thermal deaths at Yellowstone occurred in the late 1800s and early 1900s — and all were children. One woman died after the crust edge of a geyser collapsed, plunging her waist-deep into the “boiling cauldron,” she wrote in a letter from the hospital. In 1926, Whittlesey writes, a young Presbyterian pastor died from shock after he fell through several hot springs, completely submerging his head at one point. He suffered both external and internal burns from swallowing the scalding water.

Other deaths chronicled in the book included a European journalist who stumbled into a hot spring while trying to take a photograph, two park employees who mistakenly climbed into boiling springs while “hot potting,” a young boy whose parents heard a splash when he fell then watched him slink away, and a California man who dove head first into a spring in a vain attempt to rescue his friend’s dog, Moosie.

These “are the ultimate scary stories in many ways,” Whittlesey told a blogger in an interview about his book, “because they are real.”

The last recorded thermal-related death was in 2000, when 20-year-old Sarah Hulphers accidentally plunged into a hot spring with two friends as they walked through the park at night. Hulphers, a summer park employee, died 15 hours after she and the two friends were pulled from Cavern Spring, a 178-degree pool about 10 feet deep.

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After her death, some people called on Yellowstone to post more signage throughout the park, warning of the lurking dangers. But park officials said there is some behavior that signs can’t prevent. Newspaper editorials, environmentalists and park employees have long beaten the safety drum, constantly reminding the millions of visitors that pass through each year to be mindful of the power of nature.

When a child was killed after wandering into a boiling hot spring in 1970, the Billings Gazette wrote an editorial called “The Park Can Kill,” which is also quoted in Whittlesey’s book:

“Death is a frequent visitor in raw nature. And Yellowstone Park, despite the cabins and roads, is raw nature. The Park is the untamed and unfenced wildlife and the amoral energy of thermal wonders. It cannot be treated lightly; when it is it erupts in death… The park is not Disneyland, Rocky Mountain version. Nor is it a zoo with moats and fences separating the wild and the domesticated. For all the trappings of men, it is wilderness. And the man who fails to accept it as such dies.”

Scott’s death was the second thermal-related incident of the 2016 park season, officials said. On June 6, a father and son suffered burns when they walked off the designated trail in the Upper Geyser Basin.

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“We extend our sympathy to the Scott family,” Superintendent Dan Wenk said in a park statement. “This tragic event must remind all of us to follow the regulations and stay on boardwalks when visiting Yellowstone’s geyser basins.”

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Scott received his associate degree at Rogue Community College, in Oregon, according to his LinkedIn account, and earned his bachelor’s degree in psychology from Pacific University. The 23-year-old worked as a volunteer at the Jackson Bottom Wetlands Preserve, his profile said. Scott’s former boss, Mary Loftin, a manager at the Hillsboro, Ore., parks and recreation department, told the AP that he spent 20 months there, fielding questions from visitors.

“A very nice young man,” she described him. “A bright spirit.”