Last April, geologists conducting routine maintenance at temperature logging stations in Yellowstone National Park’s Norris Geyser Basin found something unexpected: a previously undocumented thermal pool of blue water.
The newly identified pool, found in the Porcelain Basin subbasin, is about 13 feet (4 meters) wide, its idyllic blue water is around 109 degrees Fahrenheit (43 degrees Celsius), and the water’s surface sits about one foot (30 centimeters) below the rim of the pool, according to a United States Geological Survey statement. The geologists found light-gray mud-covered rocks, including rocks up to one foot wide (30 cm), surrounding the pool. How did this feature form?
According to the geologists, the clues actually paint a relatively clear picture: the pool likely resulted from a hydrothermal explosion—when liquid water turns to steam and causes underground pressure changes, creating a steam blast. Hydrothermal explosions are not uncommon at Norris Geyser Basin, which has experienced similar events before. Well-documented ones include the 1989 explosion of Porkchop Geyser. More recently, a new monitoring station installed in 2023 detected an explosion in the Porcelain Terrace area on April 15, 2024.
Satellite imagery shows that the new pool did not exist before December 19, 2024. By January 6, 2025, a small cavity had begun to take shape, and on February 13, the water pool had fully formed. However, the recently installed monitoring station—which detects hydrothermal activity via infrasound (extremely low-frequency sound waves)—did not register any strong or distinct explosions during that time. It did, though, detect a number of weak acoustic signals from the direction of the pool, including on December 25, 2024, January 15, 2025, and February 11, 2025, but without an associated seismic signal that would normally accompany a strong explosion.
As such, the pool likely formed after a number of smaller explosions chucked out rocks and silica mud, as opposed to a single big event. Silica-rich water then filled the resulting hole. The activity probably started on December 25, 2024, and continued in January and early February of this year.
Norris Geyser Basin is the oldest and most active thermal area in Yellowstone and hosts the tallest geyser—a sporadically explosive hot water spring—on Earth. Yellowstone itself has over 10,000 thermal features, such as geysers, hot springs, steam vents, and mudpots, which attract tourists and scientists alike from all over the globe. The thermal activity is driven by an underground magma reservoir (part of the giant Yellowstone volcano complex), which heats up groundwater and triggers a series of chemical and physical reactions.
Ultimately, the finding shows that even the most studied landscapes can still surprise us.
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