The body of Colin Scott, 23, completely dissolved after he fell into a thermal pool about 6ft long, 4ft wide and 10ft, according to a National Park Service report into the death in June.
Scott and his sister Sable had been looking for a place to “hot pot” – the forbidden practice of soaking in one of the park’s thermal features - when he slipped.
Scott, from Portland, Oregon, had graduated with a degree in Psychology just a month before his death.
The park website describes the site where Scott died as the “hottest and most changeable thermal area in Yellowstone,” where temperatures can reach 200°F.
His sister told investigators that she and her brother left a boardwalk near Pork Chop Geyser and walked several hundred feet up a hill in search of “a place that they could potentially get into and soak,” Deputy Chief Ranger Lorant Veress told KULR-TV in an interview.
As Sable took video of her brother with her mobile phone on June 7, he reached down to check the water temperature and slipped and fell, according to a National Park Service incident record first reported by KULR.
Park officials did not release the video or a description of it, but the report said it also chronicled Sable efforts to rescue her brother.
Search and rescue rangers spotted Colin body floating in the pool on the day of the accident, but a lightning storm prevented recovery, the report said.
The next day, workers could not find any remains in the boiling, acidic water.
“In very short order, there was a significant amount of dissolving,” Veress said.
Park spokesman Charissa Reid said: “They were able to recover a few personal effects. There were no remains left to recover.
“It’s very fragile rock and can be thin as a skiff of ice.”
She added: “The rules in the park aren’t just arbitrary. They’re really here to protect people who are visiting the park and the things that they’re here to see.”
The report included images of several signs warning people of the dangers of the park’s geothermal features and of traveling off walkways in the area where Colin Scott died.
The National Park Service did not issue any citations in the case.
Scott was on a college graduation trip with his sister at the time of his death, which came a day after six people were cited for walking off-trail at the park’s Grand Prismatic Spring.
A week later, a tourist from China was fined $1,000 for breaking through the fragile crust in the Mammoth Hot Springs area, apparently to collect water for medicinal purposes.