Tourists risk being boiled alive by illegally walking right next to huge Yellowstone hot spring

Water temperatures in the spring can reach 88C, making any contact potentially fatal.

Tanya Saueressig, a resident of Livingston, Montana, saw the incident unfold while visiting the park with a friend.

She had been standing on the Grand Prismatic boardwalk at roughly 7pm on what had been an otherwise calm and enjoyable evening.

"It was a really nice crowd," she told Cowboy State Daily. "People on the boardwalk were being lovely, giving each other space, and being quiet. It was a really nice experience."

Ms Saueressig was taking a final look at the spring when she noticed movement across the vast microbial mats.

"I stopped in my tracks and was like: 'Is that people? That can't be people. Nobody's that dumb,'" she said.

It was indeed two women, who seemed to be coming from the Fairy Falls Trail along Grand Prismatic's western edge.

Her pleasant evening quickly turned sour as a rush of emotions took hold.

"I started getting scared because I thought they were going to fall in and get really, really hurt," she said.

"I worried people at the overlook were going to watch someone die that day. They were out there where nobody could help them."

Her fear soon gave way to fury as she watched the two women casually walking next to the scalding water.

"You could tell they were just strolling along, taking their selfies, without a care in the world," she said. "They were totally oblivious to what was right under their feet. You're causing so much damage with every step you take, and you don't care."

Ms Saueressig was left so angry that she was moved to tears.

"I was so angry," she said. "It was so mind-boggling to me that they would be out there in the first place."

Another visitor filmed the dark-haired pair from the overlook, with that footage amassing millions of views within a day.

It remains unclear whether the women have been identified or reported to park rangers.

Treading on the delicate microbial mats surrounding Grand Prismatic and other thermal features in Yellowstone is a criminal offence.

Those found guilty face fines, imprisonment, and a ban from the park lasting several years.

Numerous visitors have been prosecuted and sentenced for thermal trespass, yet others continue to ignore barriers and warning signs.

While there are no records of a fatal fall directly into the Grand Prismatic Spring, people have died after falling into other hydrothermal features at Yellowstone.

In 2016, 23-year-old Colin Nathaniel Scott died after falling into the waters in the park's Norris Geyser Basin. His body completely dissolved within hours.

Greg Jackson, a former deputy chief in the National Park Service's Division of Law Enforcement, Security and Emergency Services in Washington, D.C., described such visitors as "the 0.1 per cent."

"Some people see the world differently than the mainstream," he said. "You build an infrastructure that's dedicated to the 99.9 per cent of people that don't do this, and then you get people who don't see that infrastructure or decide it doesn't apply to them."