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For two centuries Yellowstone’s famous geyser has reliably erupted every 60 to 110 minutes. Geologists had thought that the spray came from a long, narrow conduit in the ground, but recent research indicates that activity in the main channel alone might not trigger the eruptions, and that a hidden underground pocket is actually key to the 130-foot-high spray.

Cameras can be lowered only so far into the geyser’s mouth because of the scalding water, so French geologists instead used seismic data collected since the 1990s. The data revealed a previously unknown side chamber 50 feet below the surface, the researchers reported in . The reservoir fills up with pressurized steam at the beginning of each cycle and “plays a major role” in how regularly the geyser erupts and how high the water sprays, says Jean Vandemeulebrouck, a geophysicist at the Université de Savoie in France and lead author of the study.

This story originally ran in the July-August 2013 issue as "Thar She Blows.”