Researchers map a hidden 79-million-gallon hot-water reservoir beneath Old Faithful

In a 2017 field campaign, researchers from the University of Utah placed 133 compact seismometers around Old Faithful and nearby Geyser Hill for a fortnight, capturing subtle ground motions generated by wind, tourists and hydrothermal activity. Using novel noise-correlation techniques, doctoral student Sin-Mei Wu and co-author Fan-Chi Lin turned these ambient vibrations into a coherent image of a hidden hydrothermal reservoir about 200 meters in diameter.

The structure can hold roughly 300,000 cubic meters, or about 79 million gallons, far exceeding the volume expelled in a single eruption. Seismic tremor patterns showed bursts lasting about an hour before each eruption, indicating that bubble implosions refill the cavity. The findings aid Yellowstone Volcano Observatory and the National Park Service in managing heat flow beneath visitor facilities and demonstrate a low-impact method for imaging shallow geology in sensitive settings.