That’s good news for the people of the Tibetan plateau, hundreds of thousands of whom harvest the fungus as their main source of income. It pays for food, clothes, medical bills, and education. It allows them to eke out a living on the roof of the world, where a living is increasingly hard to eke out.

But tough times lie ahead. By interviewing hundreds of collectors, and analyzing the local climate, Hopping has conclusively shown what others have suspected: The precious fungus is disappearing, as a result of a double whammy of overharvesting and warming weather. The caterpillar-fungus bubble is ready to burst, and an entire way of life could vanish with it. “I asked them, ‘Would you do something different if you could?’” Hopping says. “A lot of people said, ‘Yes, if there was another way to make money. But I don’t have any other options.’”

The fungus first infects caterpillars in the summer, while they are buried underground and feeding on plant roots. It grows through their bodies in the fall and winter, slowly consuming them. Once the overlying snow melts in the spring, the fungus forces its almost-dead hosts toward the surface, before sending a dark-brown, spore-filled stalk through their heads. For that reason, the fungus is known locally as yartsa gunbu, from the Tibetan words for “winter worm, summer grass.”

Spotting the dark stalks that the fungus extrudes is tricky, since they closely resemble the sedges that flourish in Tibet’s alpine meadows. Digging them out is even harder. “If the fungus breaks off the caterpillar, it loses value, so during the harvest, it’s really important to keep the two parts connected,” says Hopping. “And the turf in these areas is so thick that when I was taking soil samples, I’d have to cut it with a knife.” It’s laborious work, but for the collectors, it’s also a social activity—a chance to hang out with friends on a summery mountainside.

The collectors work in May and June. Once they’ve unearthed their prizes, they sell to visiting traders using a complicated haggling system, in which their hands secretly exchange offers and counteroffers while hidden by the long sleeves of their robes. Once sold, the caterpillars slowly work their way through a chain of middlemen and toward the bustling metropolises of Hong Kong and mainland China.

Though the fungus has been prized as medicine for centuries, demand for it was long restricted to elites. That started changing in September 1993, when a team of Chinese athletes unexpectedly smashed several world records in track-and-field events—a feat that their coach partly attributed to their consumption of caterpillar fungus. Whether or not that was true (and there have long been suspicions about doping), the claims helped to turn the fungus into a valuable commodity, as did subsequent rumors in 2003 that it could help ward off SARS. Demand surged, and collecting and selling the weird parasite became truly profitable.