Source: Waipapa Taumata Rau, University of Auckland
MONDAY, AUGUST 17, 2026
University of Auckland researchers have conducted what may be the world’s deepest physiological study of freedivers, helping explain blackouts and other effects of extreme depth.
Research into why freedivers black out and experience unusual symptoms at extreme depths has been recognised by the American Physiological Society as one of the world’s top recently published physiology papers.
The pioneering study collected arterial blood samples from elite freedivers at depths of up to 80 metres during open-water dives, offering rare insight into how the body responds to extreme physiological stress.
Professor Simon Mitchell, head of anaesthesiology at Waipapa Taumata Rau, University of Auckland, and consultant anaesthesiologist Dr Tom Scott, led the ambitious expedition to Dominica in the eastern Caribbean, backed by Inkfish Expeditions. Bringing together researchers, elite freedivers and specialist mixed-gas divers, the mission investigated how the body responds to extreme breath-hold diving.
“No one had ever collected arterial blood samples during dives like these before,” Mitchell says.
“What makes this study remarkable is how difficult it was to pull off. This project required multiple areas of expertise to come together for it to work.”
Mitchell has been named the 2026 recipient of the prestigious NOGI Award for Science, recognising his contributions to diving science.
The American Physiological Society has also selected his paper, ‘Arterial blood gas changes in progressively deeper breath-hold dives’, for APSselect, a monthly showcase of outstanding physiology research published in the society’s journals.
The research examined three elite freedivers, including University of Auckland alumnus and world champion William Trubridge, the first person to dive deeper than 100 metres without changing his weighting or using fins.
The study unveiled three major findings.
“Carbon dioxide levels go extremely high during these deep dives, high enough to explain many of the symptoms freedivers experience at depth,” Mitchell says.
The team also found the reason for the sudden loss of consciousness some freedivers experience near the surface.
“We showed that oxygen levels drop extremely quickly as divers return to the surface and their lungs expand. That’s why blackouts can happen so suddenly, with almost no warning,” he says.
A third, unexpected finding was a previously unknown physiological phenomenon. Rather than continuing to rise as the lungs are compressed during descent, oxygen levels peaked at about 40 metres before declining during deeper descent.
“Nobody knew that occurred, before this study,” Mitchell says.
The findings could help make freediving safer.
“Our work explains why those blackouts occur and serves as a reminder not to put yourself in situations where they’re more likely,” he says.
“It’s important to freedive very conservatively and never push beyond your limits. The deeper you go, and the longer you stay at depth, the higher the risk.”
The research suggests the unusual sensations elite competitive freedivers experience at extreme depths may stem from carbon dioxide impairing brain function.
“We at least have an explanation for it now, and it should remind people not to push deeper if they’re getting those kinds of symptoms,” he says.
Blood samples were collected at 20, 40, 60 and 80 metres to track how oxygen and carbon dioxide levels changed during breath-hold dives.
“The project depended on three areas of expertise: the freedivers’ ability to complete the dives, the mixed-gas divers, including me, who could spend four hours underwater day after day, and the extensive surface support that made the entire operation possible,” he says.
Among the expedition’s key personnel was Dr Richard Harris, the Australian anaesthetist and cave diver who helped rescue 12 boys and their football coach from Thailand’s flooded Tham Luang cave system in 2018. Harris joined the team as one of the specialist mixed-gas divers responsible for collecting blood samples at depth.
The study was inspired by Professor Mike Grocott’s landmark research, which measured arterial blood gases in climbers at extreme altitude, including near the summit of Mount Everest.
“I read that Everest paper and thought, ‘What if we could do the same thing in freediving?’ Nobody had done it before,” Mitchell says.
The project team is planning a return to Dominica next year to continue investigating the effects of extreme carbon dioxide levels and explore whether additional safety recommendations can be developed for freedivers, he says.
A mini-documentary released on YouTube follows the expedition and captures the challenges of conducting science in one of the world’s most demanding environments.
Mitchell is quick to share the credit.
“We had an amazing team from the University and extraordinary support from Inkfish,” he says.
Read the study
See the mini-documentary