Jeju's Haenyeo women have spent generations diving without oxygen tanks; a 2025 study found their hearts slow dramatically underwater, likely shaped by years of training

Off the rocky coast of Jeju Island in South Korea, women in their sixties, seventies and even eighties continue to dive into the sea without oxygen tanks. They are the Haenyeo, generations of women who harvest abalone, sea urchins and seaweed from the ocean floor while holding their breath. A paper titled Genetic and training adaptations in the Haenyeo divers of Jeju, Korea, by Diana Aguilar-Gómez and colleagues, was published in Cell Reports on May 27, 2025. The study examined physiological and genetic adaptations associated with breath-hold diving among the Haenyeo.

The word Haenyeo translates loosely as 'sea women'. These divers typically descend to around ten metres and remain underwater for roughly thirty seconds at a time, repeating the dives during several hours of work. Some Haenyeo have also continued diving during pregnancy, reflecting how deeply the practice is woven into life on Jeju. Yet the tradition is declining. The current Haenyeo population has an average age of around seventy, and young women are no longer continuing the tradition, leaving the community increasingly dominated by older divers.

Anyone who holds their breath and submerges their face in cold water can experience the mammalian dive reflex. This automatic response slows the heart and constricts blood vessels near the skin, helping direct oxygenated blood towards vital organs such as the heart and brain. Marine mammals, including whales, seals and dolphins, rely on a much stronger version of the same oxygen-conserving response, while humans display it to a lesser degree.


A heart that learns to slow right down

To understand what sets the Haenyeo apart, the researchers ran a series of simulated dives with three groups of women: the Haenyeo themselves, non-diving residents of Jeju, and women from mainland Korea. Each participant held her breath and submerged her face in cold water while heart rate and blood pressure were tracked throughout.

All three groups showed the expected drop in heart rate, confirming that the dive response was active in everyone. The Haenyeo, however, showed a far steeper decline. Their heart rate fell by 18.8 beats per minute on average, compared with 12.6 beats per minute among the non-diving Jeju women. The difference was statistically significant, and the Haenyeo achieved it despite having shorter simulated dives, averaging 27 seconds compared with 39 seconds for the Jeju controls.

The researchers found no clear genetic distinction between the Haenyeo and non-diving Jeju women that could explain this stronger response. They therefore concluded that the difference was likely linked to the Haenyeo's lifelong diving training, suggesting that years of repeated breath-hold dives had shaped how their bodies respond underwater.

The genetic findings emerged instead when the researchers compared Jeju Islanders with people from mainland Korea. Jeju residents, divers and non-divers alike, carried certain variants at different frequencies from mainland participants. One selected variant, in the gene encoding sarcoglycan zeta, has previously been associated with pain sensitivity during exposure to cold water. The researchers suggested it may be related to the Haenyeo's ability to tolerate cold, although they did not directly measure thermoregulatory responses in this study. Another variant was associated with lower diastolic blood pressure and was found in 33% of Jeju participants, compared with 7% of those from mainland Korea.

Why the blood pressure link matters, especially in pregnancy

That blood pressure connection is particularly important because Haenyeo have traditionally continued diving during pregnancy. Breath-hold diving triggers vasoconstriction and can sharply increase blood pressure, while elevated diastolic blood pressure is associated with pregnancy complications including preeclampsia. The researchers suggest that natural selection may have favoured the Jeju variant because its association with lower diastolic blood pressure could help mitigate these effects in women who continued diving while pregnant.

Beyond the Haenyeo community, the finding could have wider relevance. The researchers note that preeclampsia is also associated with long-term cardiovascular health risks, raising the possibility that this genetic variant and its effect on blood pressure could provide useful clues for future research. However, the study did not establish that the variant prevents pregnancy complications or improves cardiovascular outcomes.

None of this means people should try to replicate the Haenyeo's abilities by holding their breath or practising breath-hold diving in cold water on their own. Breath-hold diving can cause loss of consciousness and other serious dangers, and should only be undertaken with appropriate training and professional supervision. The Haenyeo study instead provides a striking example of how specialised training over many years can alter the body's response to a demanding physical activity.

The findings also come as the Haenyeo tradition faces a shrinking and ageing workforce. By documenting the physiological effects of a lifetime spent diving, the study provides a detailed look at how a distinctive way of life can become reflected in the human body.