The hydrothermal explosion happened around 10 a.m. in Biscuit Basin, a collection of hot springs a couple miles north of the famous Old Faithful Geyser.

Video posted online showed a couple dozen people watching from a boardwalk as the eruption sprayed and grew in front of them. As water and debris began to fall, they ran to keep clear, some yelling "Back up!" and "Holy cow!" People then turned to watch the spectacle under a huge cloud of steam.

No injuries were reported, but the Biscuit Basin area was closed for visitor safety. The eruption damaged a boardwalk that keeps people off Yellowstone's fragile and often dangerous geothermal areas.

Vlada March was on a tour in the basin when her guide said something unusual was happening. March started taking video.

"We saw more steam coming up and within seconds it became this huge thing," said March, a California real estate agent who was with her mom, husband and their two kids. "It just exploded and became like a black cloud that covered the sun."

"I think our tour guide said 'Run,' and I started running and I started screaming at the kids, 'Run, run, run,' and I continued filming what I could," she said.

Rocks that fell from the sky smashed the boardwalk they had been walking on. March's mom, who had been sitting on a bench near the explosion, was shaken and dirty but otherwise fine, she said.