Invisible volcanic gas at a California ski resort has killed three ski patrollers

Charles Rosenthal, James Juarez and John “Scott” McAndrews died on Mammoth Mountain on April 6, 2006, not from an avalanche or a fall, but from carbon dioxide gas that had pooled invisibly beneath the snow. The three ski patrollers had been fencing off a known volcanic vent on the Christmas Bowl run when the snowpack over it collapsed, dropping two of them into the pit; a third died attempting to rescue them. The gas came from Horseshoe Lake, a spot at the base of the resort that sits directly above the Long Valley Caldera, a dormant supervolcano the U.S. Geological Survey still classifies as active today.

Nearly two decades later, the same hazard that killed them has not gone away — it has only been better mapped and posted with warning signs.

Three Names on a Mountaintop Monument

Rosenthal was 58, Juarez was 35 and McAndrews was 37. All three were experienced patrollers inspecting the mountain after heavy snowstorms, part of a four-person team assigned to re-fence a fumarole that had been temporarily buried and blocked by snowfall. When the snow gave way, two patrollers fell roughly 21 feet into a hole about 6 feet across; the volcanic gas that had built up beneath the blocked vent asphyxiated them before rescue was possible. Mammoth Mountain later placed a permanent stone monument, designed by local artist Larry Walker, at the resort’s summit in their memory.

A Cal/OSHA investigation that followed found the resort had no procedure for evaluating fumarole hazards and had not trained rescue or medical staff to recognize dangerous gas buildup. The agency issued $49,865 in fines. Len Welsh, Cal/OSHA’s acting chief at the time, did not mince the finding: “If standard practices had been followed, this catastrophic event might not have occurred.”

The Chemistry Underneath Horseshoe Lake

What makes the hazard so hard to see coming is the gas itself. Carbon dioxide has no color and no smell, and it is heavier than air, so it collects in low, enclosed spaces like snow pits and depressions rather than dispersing. In the affected soil around Horseshoe Lake, the USGS’s own field guide to the site puts carbon dioxide at 20% to 95% of total soil gas, against 1% or less in normal ground. Breathing air with more than 7% CO2 can cause unconsciousness and death within minutes, which is roughly the concentration a person could encounter in the worst pockets around the lake.

The gas had already announced itself before 2006. U.S. Forest Service rangers noticed dead and dying trees spreading across more than 100 acres around Mammoth Mountain starting around 1990, and a USGS soil-gas survey confirmed volcanic carbon dioxide as the cause in 1994. The trees were dying from the roots up, poisoned by gas seeping through soil that still looked, from the surface, like ordinary Sierra Nevada forest floor.

A Caldera That Rose Nearly Two and a Half Feet

The gas is a symptom of something larger moving underground. In May 1980, four magnitude-6 earthquakes struck the Long Valley area in a single day, and USGS scientists who went back to examine the caldera afterward found the ground had begun swelling. By the early 2000s, the USGS’s own summary of the uplift put the total rise at nearly 2.5 feet across an area exceeding 100 square miles, at a pace of roughly an inch a year, driven by magma moving beneath the resurgent dome at the caldera’s center.

USGS still ranks Long Valley Caldera among the nation’s small group of “Very High” threat volcanic systems, a 20-by-32-kilometer caldera in Mono County that the agency describes as remaining thermally active, with ongoing seismicity, deformation and hot springs and fumaroles it monitors through a dense network of field sensors. The same geothermal heat that vents as dangerous gas at Horseshoe Lake also powers the Casa Diablo plant nearby, which supplies electricity to roughly 40,000 homes.

What Has and Hasn’t Changed Since 2006

CO2 flux around Horseshoe Lake has declined gradually over the past several years, according to USGS’s monitoring, though the agency still describes the high-flux areas around the lake as an ongoing hazard rather than a resolved one. Fencing, signage and public warnings now mark the ground the three patrollers were working on when they died, the direct legacy of what the Cal/OSHA investigation found missing in 2006.

What has not changed is the underlying geology. The caldera that vents the gas is the same one that lifted nearly 2.5 feet after the 1980 earthquake swarm, and USGS has not downgraded its threat classification since. Horseshoe Lake remains one of the only places in the country where a stretch of ordinary-looking ground, a few feet of snow, and no smell or color to warn anyone, is still capable of the same outcome it produced in 2006.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.