Old Faithful, it turns out, wasn’t always so faithful. The geyser, in Wyoming’s Yellowstone National Park, is famous because it blasts hot water tens of meters into the air at regular intervals—every 90 to 94 minutes, on average. Now, geologists examining petrified wood from the park have found evidence that 800 years ago, Old Faithful stopped erupting entirely for several decades, in response to a severe drought. With climate change making drought more common across the western United States, the researchers say a similar shut down might happen again.

The world’s 1000 or so geysers typically occur in areas that are volcanically active, like Yellowstone. Water percolating down through the ground reaches the boiling point as it approaches the heat of a magma chamber. But because the water is deep underground it is also at a high pressure that prevents it from becoming steam. Eventually the superheated water becomes hot enough to vaporize, triggering an explosive eruption of water and steam at the geyser’s vent.

Many geysers erupt randomly, but when Henry Washburn and his fellow explorers traveled through Yellowstone in 1870, they noted the regularity of Old Faithful, and named it to reflect its predictability.

Shaul Hurwitz, a geologist at the U.S. Geological Survey, and his colleagues wondered whether it was always so dependable. Today, the area around the geyser is barren because vegetation can’t survive the blasts of hot, alkaline water. Back in the 1950s, however, a researcher found chunks of ancient wood near the geyser, mineralized and preserved by Old Faithful’s silica-rich waters. The wood suggested there was a time when trees could grow because there were no eruptions. To investigate, Hurwitz and his colleagues persuaded the National Park Service to let them date 13 such specimens.

“When I submitted the samples for radiocarbon dating I didn’t know whether they would be hundreds or thousands of years old,” Hurwitz says. “It was an ‘aha!’ moment when they all clustered within a hundred-year period in the 13th and 14th centuries.”

Three of the wood fragments were lodgepole pines, still common in Yellowstone today, the team reported on 7 October in Geophysical Research Letters. One specimen—a 2.4-meter-long section of trunk—came from a tree the researchers estimate grew for 80 years, which suggests Old Faithful failed to erupt for almost a century.

The researchers turned to the scientific literature in search of an explanation for the eruption hiatus. They found that all their samples fell at the end of the Medieval Climatic Anomaly, a warm and dry period felt across the Northern Hemisphere thought to have been caused, in part, by changes in ocean circulation. It began in 950 C.E. and in the western United States was still causing droughts in the late 1200s. The researchers argue a lengthy drought would have starved Old Faithful of the water it needs to erupt, paradoxically allowing some drought-tolerant trees to grow.

Jamie Farrell, a geophysicist at the University of Utah who studies Yellowstone and who wasn’t involved in the study, says the analysis makes sense. “If you have prolonged drought and there isn’t enough water to feed these systems, then features like Old Faithful might sometimes stop erupting,” he says.

Broxton Bird, a geologist at Indiana University–Purdue University, Indianapolis, who specializes in climate change, agrees. He says the Medieval Climatic Anomaly was severe enough to make it happen. “Water tables would be getting lower and lower, and right at the end of that Old Faithful shuts down,” he says. The droughts would have made farming in the western United States difficult, he adds. This may help explain why the Pueblo dwellings of ancestral Native Americans in the southwest United States were abandoned at the time.

Hurwitz and his colleagues say that with climate change making megadroughts more likely across the western United States, Old Faithful might erupt less frequently in the future–and might even stop altogether.

Maxwell Rudolph, a geophysicist at the University of California, Davis, says that would be tragic for the millions of people who visit Old Faithful. “The extinction of this natural treasure would be a profound loss.”