CHEYENNE, Wyo. — A surprise eruption that shot steam, water and dark-colored rock and dirt dozens of feet into the sky Tuesday sent people running for safety in Yellowstone National Park.

The hydrothermal explosion happened around 10 a.m. in Biscuit Basin, a collection of hot springs a couple miles (3.2 kilometers) 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.

The eruption damaged the boardwalk, an elevated wooden walkway that keeps people off Yellowstone’s fragile and often dangerous geothermal areas. Photos and video of the aftermath showed damaged guardrails and boards covered in rock and silt near muddy pools.

No injuries were reported, but the Biscuit Basin area was closed for visitor safety, according to a U.S. Geological Survey statement.

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Explosion Occurred Near Black Diamond Pool, Enlarged It

The explosion happened in or near Black Diamond Pool, a 120-foot-long (37-meter-long) hot pool that is the basin’s widest thermal feature.

A National Park Service aerial photo taken afterward showed the pool somewhat enlarged near the boardwalk compared to recent satellite images and its water turned a muddy beige. The deep Sapphire Pool about 100 yards (91 meters) away retained its usual blue hues.

A hydrothermal explosion happens when water suddenly flashes to steam underground. Such blasts are relatively common in Yellowstone. Park geologists were investigating what specifically happened in this case.

Similar blasts have happened in Biscuit Basin in 2009, 1991 and after the magnitude 7.2 Hebgen Lake earthquake 40 miles (64 kilometers) away in 1959.

Dramatic as it was, the latest was on the small side, according to the statement.

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Hydrothermal Explosions Have Shaped Yellowstone’s Landscape

Scientists theorize that a series of hydrothermal explosions created Mary Bay on the northeastern side of Yellowstone Lake some 13,800 years ago. At 1.5 miles (2.4 kilometers) wide, Mary Bay is the world’s largest known hydrothermal explosion crater.

Yellowstone is centered on a huge, dormant volcano. The hydrothermal explosion did not indicate new activity within the volcanic system, which remains at normal levels, according to the Geological Survey.

Though no one was hurt, that’s not always the case with Yellowstone’s thermal features.

At least 22 people are known to have died from injuries related to thermal features in and around the 3,471-square-mile (9,000-square-kilometer) national park since 1890.

Visitors are warned to stay on the boardwalks and trails in thermal areas, where some of the pools and springs have a thin, breakable crust covering the scalding and sometimes acidic water.