A group of Kiwi scientists have pulled off a world first, discovering the reason for freedivers to suffer shallow water blackouts.
Research done in the depths of Lake Taupō has shown that despite a diver's body using oxygen as they descend, the compression on their lungs means their oxygen levels can triple.
"It creates a sense when you're on the bottom that you feel comfortable because your oxygen levels are quite high," head of anesthesiology at the University of Auckland's Dr Simon Mitchell explained.
"But then as you come to the surface and your lungs expand and your oxygen levels fall very quickly, it creates this problem of potentially blacking out."
"There's always been this education around why free divers and spear fishermen are at risk of blacking out but there's never been the science or data to back up why that actually occurs," Dr Tom Scott told 1 NEWS.
That is no surprise, when one understands the difficulty in obtaining said data.
To work out what actually happens to a diver's oxygen levels Scott had to find a way to take a diver's blood underwater, and then again when they resurface before they take a breath.
"Going into the weekend I had probably about 40 per cent confidence that we really pull it off, there were so many moving parts that needed to really go well to get the results we needed. I was nervous," he said.
Mitchell was the one tasked with taking the blood samples, 60 metres below the surface.
"It was so cold, it was ten degrees. My hands were quite cold and I kind of fumbled... I could hardly feel what I was doing," Mitchell said.
But it was all worth it in the end. The research will be used for education, to ensure what goes down, comes up safely.
"It is important that the average recreational diver understands that this applies to the dives they do," Mitchell said.