Some people are just built differently. In the case of the Haenyeo [hen-yeo], however, the formula is a little nature, a little nurture and a whole lot of cold water.

The Haenyeo are all-female divers from Jeju [jee-joo] Island, South Korea, known for freediving in frigid seas without any breathing gear. Sometimes, they even do it during pregnancy. A new study looked at why their bodies can do this, and found the answer lies in their DNA — and a lifetime of practice.

Researchers compared 30 Haenyeo divers with similarly aged women from Jeju and mainland Korea, measuring heart rate and blood pressure during “simulated dives.” In these simulations, participants were asked to hold their breath while dunking their face in cold water as a stand-in for what the Haenyeo typically experience during one of their excursions.

Everyone’s heart rate slowed down, but the Haenyeos’ dropped much more, decreasing by nearly 19 beats per minute, on average. One diver’s heart rate fell by over 40 bpm in just 15 seconds.

That heart-slowing reflex, known as bradycardia, seems to come from decades of daily diving. But genetics plays a role as well. Notably, two gene variants stood out in the Haenyeo: one linked to cold tolerance and one to lower blood pressure. Both could help protect against the physical strain of diving, especially during pregnancy.

According to the study, all Jeju islanders, divers or not, seem to share a distinct genetic background. However, only the Haenyeo push their bodies to these frigid limits.

The takeaway? Evolution and endurance make a powerful pair — especially when your job is at the bottom of the sea.