In May 1999, 29-year-old radiologist Anna Bågenholm and two other young doctors set out for a day of skiing in the Kjolen Mountains of Norway.

All experienced skiers, they headed off track to enjoy the fresh powder, but Bågenholm took a fall and slid downhill, landing head-first through the ice of a frozen stream. 

Her friends quickly grabbed hold of her skis and tried to pull her out, but the ice was too thick and she was jammed between rocks. They called for help, but in the meantime, Bågenholm was stuck upside down, in heavy gear, and being pulled in by icy cold water - pretty much a worst-case scenario. 

Thankfully, she managed to find a pocket of air, and struggled to stay conscious waiting for help to arrive, but after 40 minutes, she stopped moving. By the time the rescue team arrived, she had been under the ice for 80 minutes, and was frozen solid.

Her heart wasn't beating, she wasn't breathing. She was clinically dead. No one had ever been brought back from such a low temperature before, but her friends immediately started CPR on her, hoping that she might be able to be revived after being air-lifted to the University Hospital of North Norway in Tromsø.

By the time she reached the operating room at the hospital, it had been more than 2.5 hours since she first fell in the ice, and her temperature was still an unprecedented 13.7 degrees Celsius (56.7 Fahrenheit). 

"She has completely dilated pupils. She is ashen, flaxen white. She's wet. She's ice cold when I touch her skin, and she looks absolutely dead," head of the emergency medical department, Mads Gilbert, told CNN a decade later.  

"On the ECG [electrocardiogram], which the doctor on the helicopter has connected her to, there is a completely flat line. Like you could have drawn it with a ruler. No signs of life whatsoever."

But he made an important decision. "We will not declare her dead until she is warm and dead," said Gilbert.

Gilbert's hope was that Bågenholm was so cold that her brain would have begun to slow down before she died, protecting it from damage.

At our bodies' preferred temperature of 37.5 degrees Celsius (99.5 Fahrenheit), our brains can only go for around 20 minutes without oxygen before irreversible damage sets in. But as we cool down, the body's metabolism also slows way down in order to keep us alive, which means the brain can get by with far less oxygen.

Given how unprecedentedly cold Bågenholm was, Gilbert thought that perhaps the continuous CPR she'd received since being pulled out of the frozen river might have been enough to keep her brain functioning properly. 

His team hooked Bågenholm up to a heart-lung machine, and they pumped her blood out of her body to warm up before circulating it through her again - very gradually, over several hours, her temperature began to rise.

The heart monitor blipped a few times, but continued to flatline. The team waited. 

At around 4pm the next day, almost a day after Bågenholm had fallen into the river, her heart restarted, and began pumping the blood on its own again.

Gradually the rest of her body began to heal, too. After 12 days, she opened her eyes, but it took more than a year for her to be able to move and walk again due to nerve damage.

She's now fully recovered, and works at the same hospital that saved her life. 

Her case didn't only make the record books, and a study in The Lancet journal - it also changed the way doctors approached hypothermia deaths.

"In a victim of very deep accidental hypothermia, nine hours of resuscitation and stabilisation led to good physical and mental recovery. This potential outcome should be borne in mind for all such victims," concluded The Lancet study.

Before 1999, no patient had survived being frozen to death at the University Hospital of North Norway. But between 1999 and 2013, nine out of 24 patients survived hypothermic cardiac arrest, a 2014 study led by Gilbert found. 

Doctors at the University of Pittsburgh Medical Centre hospital are now actually inducing hypothermia in critical patients, to prolong the window in which they can stop their bleeding and save their lives. 

"We think of death as being a moment in time," physiologist Kevin Fong told NPR back in 2014, "but actually, it is a process."

When we're warm, that process happens in minutes, but when we're cold it slows down - and, occasionally, that can work in our favour.

"It's the three important things about emergency medicine, which is never give up, never give up, never give up," Torvind Næsheim, one of the doctors who was skiing with Bågenholm that day told CNN. "Because there's always hope."

H/T Atlas Obscura