Yellowstone National Park's Steamboat geyser - the world's tallest - has erupted for the first time in more than eight years.
Park geologist Hank Heasler said the nine-minute blast sent steaming hot water an estimated 300ft in the air.
Unlike Yellowstone's Old Faithful geyser, which spews water like clockwork every hour-and-a-half, no-one knows when Steamboat will erupt next.
In the past, it has gone as long as 50 years without a major event. In 1964, it erupted a record 29 times.
Steamboat is one of more than 500 geysers at Yellowstone, which boasts the largest collection of hydrothermal features in the world. They are fuelled by cold water seeping into a natural underground plumbing network, where heat from the park's volcano causes the periodic eruptions.