Thousands of tourists, park rangers and firefighters were battling a massive fireball in the Yellowstone National Monument, just a few hours after a thunderstorm blew through the region.
The eruption of a huge fireball that left trails of ash, gas and water across the scenic landscape, where people are camping, and in the nearby town of Big Bear, Montana, was the biggest such event in US history.
“It’s really cool to see people on the ground,” said park ranger David Brown, who was on the scene of the explosion.
“We are just seeing the beauty of the outdoors.”
Brown said there was no sign of the fire’s initial explosion.
But it soon spread rapidly, with the blaze burning for several kilometres, with a gust of up to 250km/h (150 miles/hour).
It was not immediately clear what caused the fire to explode, but the National Park Service (NPS) said it could have been caused by a lightning strike or an electrical short.
“This was a major fire, but it is a very small fire,” Brown said.
“The fireball was huge, it was about 40 metres (150 feet) across.
It had a trail of ash and it was hot.
It was like a tornado.”
Brown added that firefighters could not see the fire for several miles, and that many of them had to call in a helicopter to try and get to the scene.
“That was a big relief,” he said.
There were no immediate reports of injuries, and the NPS said it had no information on the cause of the blast.
Yellowstone National Park, a national park in western Montana, covers more than 10,000 square kilometres (3,000 sq miles).
The park’s superintendent, Dave McManus, said the fire was expected to last several days.
“We expect it to be contained by tomorrow morning,” he told the Associated Press news agency.
“The fire is so huge that it’s almost impossible to get to.
It will be burning for at least a few days.”
The Yellowstone National Forest is about 200 kilometres (124 miles) south of Yellowstone.