'Blast of rock, water and steam': Hydrothermal explosion rocks Yellowstone National Park, sends dozens running for safety

The hydrothermal explosion happened around 10 am 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.