Earlier this month, U.K. mountaineer Richard Parks prematurely abandoned his team’s expedition to the summit of Mount Everest in Nepal. He planned to ascend the peak without supplemental oxygen as part of Project Everest Cynllun, and take the highest-elevation blood sample and muscle biopsy ever collected. The project’s original goal was to examine the link between hypoxia and cognitive decline by examining human performance in low-oxygen environments, but its abrupt end has sparked questions of a different sort.

The team had for several weeks been climbing smaller peaks to acclimatize to high altitude, and Parks was about to start his second rotation up the mountain: a 2-week stay above the Khumbu Icefall (5486 meters). Damian Bailey, a physiologist at the University of South Wales in the United Kingdom and the lead scientist on the project, decided to perform a blood test on Parks earlier than scheduled. When he drew the blood, he immediately knew something was wrong. “His blood was extraordinarily thick,” Bailey says. “It was actually clotting as I was taking a sample.”

Testing revealed that Parks had exceptionally high levels of red blood cells and a high hematocrit, the percentage of the blood’s mass made up of red blood cells. On one hand, this was a clue to Parks’s ability to function in low-oxygen conditions: “His brain was actually getting more oxygen than it would get at sea level,” Bailey says, despite the thin alpine atmosphere containing half the amount of oxygen found at lower elevations. But such high cell densities also put him at increased risk of a stroke or a heart attack. For this reason, the team decided to end the expedition on 3 May despite Parks outwardly seeming in perfectly good health.

The project still managed to collect data for its original goal of examining the link between hypoxia and cognitive decline. Plus, Bailey hopes to plan a follow-up expedition at some point. But for now, there is more testing to be done back in the lab—and Parks’s unusual physiology has turned out to have implications not just for cognitive decline, but also more directly for would-be Everest climbers.

Parks’s exceptionally strong response to altitude suggests that climbers could “overacclimatize,” Bailey says, or put themselves and others in danger by spending too much time at high altitudes in preparation for an ascent. That runs against the prevailing wisdom that thorough acclimatization is a necessity for any ascent without supplemental oxygen.

And given Parks’s outward signs of good health, it’s difficult to know how widespread the problem might be: How many climbers are already experiencing these symptoms without knowing it? And should high-altitude specialists be testing for this on the mountain? These aren’t the questions the team meant to provoke—but that doesn’t mean they’re unwelcome. “This is the dynamic of science,” Bailey says. “You can stumble across some findings … that can be just as profound as the original question you wanted to answer.”