Great Fountain Geyser, Fountain geyser in Lower Geyser Basin, Yellowstone National Park, United States.

The Great Fountain Geyser is a geyser in the Lower Geyser Basin of Yellowstone National Park that shoots water up to 220 feet (67 meters) high from a single opening. The eruption unfolds in multiple bursts that last around one hour in total.

In 1969 researchers from Indiana University isolated the thermophilic bacterium Thermus aquaticus from Mushroom Pool, a non-eruptive hot spring a few hundred feet from nearby White Dome Geyser. Its DNA polymerase later became central to the polymerase chain reaction, the laboratory technique used to amplify DNA. The geyser itself is the only feature in the Lower Geyser Basin for which the park issues eruption predictions.

The geyser sits in the Firehole Lake area, and its parking lot doubles as the viewpoint for both Great Fountain and the nearby White Dome Geyser, which erupts more often but less powerfully from a large geyserite cone. Rangers post eruption predictions for Great Fountain, the only Lower Geyser Basin feature the park forecasts.

A dedicated parking lot serves as the viewpoint for the geyser and also overlooks White Dome Geyser nearby. Predictions are refined to about plus or minus 15 minutes by watching overflow from the crater between eruptions. During wild-phase episodes eruptions become unpredictable and intervals can reach three days.

The bacterium Thermus aquaticus was discovered near this geyser and became crucial to transforming how scientists analyze and study DNA. This tiny organism thriving in extremely hot water led to laboratory techniques that are now used in research facilities worldwide.