Last February, Justin Smith was found lying face down in a foot of snow. He wasn’t breathing, had no pulse or blood pressure, and his body temperature was below 20 degrees Celsius. According to the vital signs, Smith was clinically dead—and had been for 12 hours.

But when paramedics at the scene called to Dr. Gerald Coleman, from Lehigh Valley Hospital-Hazleton, they was told to start CPR. Coleman told Standard Speaker, “My clinical thought is very simple: You have to be warm to be dead.”

Smith was given CPR for 15 hours, pumped with warm oxygenated blood, and woke from his coma two weeks later. Less than six weeks after Smith was found “dead,” he was released from hospital. He lost his toes and pinkie fingers to frostbite, but was otherwise unharmed.

This is not the first time someone has gone into hypothermia, lost all signs of life, and survived. Reduced body temperature means that the body’s cells have a slower metabolic rate, and so they need less energy and can survive without the breathing and blood flow usually needed to live.

“Hypothermia suppresses metabolism. If it happens rapidly and significantly enough, the brain doesn’t suffer irreversible damage,” H. Craig Heller, biology professor at Stanford University tells Quartz. “But if it’s not enough or happens too slowly, then the brain can get to the point of no return.”

Putting hypothermia to use in medicine

These cases of accidental hypothermia and miraculous-sounding survival stories have inspired doctors to embrace the potential life-saving benefits of cooling down.

Currently, doctors reduce body temperature slightly (by a few degrees, nowhere near as extreme as in accidental hypothermia) to treat premature babies and brain injury.

But after animal trials, where pigs were cooled to 10 Celsius and survived, researchers are conducting experiments on therapeutic hypothermia for humans. The trial, which began in 2014 and is currently being conducted in the University of Pittsburgh Medical Center, is used on patients who have suffered severe trauma such as gunshot wounds. Patients are cooled to give doctors more time to treat their wounds and suspend the dying process. And, in order to make the cooling process as quick as possible, the patients’ blood is pumped out and replaced with a cold saline solution.

“The only reason we use cold saline is that we found it’s the fastest way to cool the brain and other organs,” Samuel Tisherman, the surgeon leading the trial, tells Quartz. “The patients have already lost more than half their blood volume because of trauma before we start the saline.”

“When do you declare a person dead—is it brain death, is it heart death?”

Once the injuries are treated, the saline solution is then again replaced with blood and the patient is warmed up. The trial can only be conducted on patients who are expected to die, which means it takes longer than usual to collect definitive results. Tisherman says the trial results are unlikely to be published for at least another two years.

But such work has potential implications for how to determine whether someone is “dead.” Tisherman is unphilosophical about this. “Patients are dead when we have run out of ways to save them and we declare them dead,” he says.

But others acknowledge that it’s increasingly difficult to draw a stark line between life and death. “When do you declare a person dead—is it brain death, is it heart death?” says Heller. “In actuality we’re measuring cellular death. But we’re talking about trillions of cells in humans. What percentage has to die for someone to be dead?”

Using hypothermia to induce long-term hibernation

Hypothermia is likely to become increasingly used in medicine over the coming years. But over the coming decades, scientists have an even more ambitious goal: To induce a long-term state of regulated hypothermia, or torpor, to allow humans to hibernate.

Hibernation essentially allows mammals, many of which have similar body temperatures to humans, to massively reduce their body temperatures and survive. And human hibernation has attracted the interest of the European Space Agency, which has long-term hopes of putting astronauts into hibernation for lengthy space travel.

“We see the science has advanced enough to put some of the science fiction into the realm of science reality,” Leopold Summerer, head of advanced concepts team of the European Space Agency, told the Washington Post.

ESA is currently working with scientists around the world to investigate the potential for hibernation in humans. But first scientists have to figure out the detailed mechanisms behind hibernation.

“Hibernation is a very complex, well-integrated physiological response that we would hope to be able to mimic with drugs in humans,” Kelly Drew, a biochemist at the University of Alaska at Fairbanks tells Quartz. “We’re a long way off from understanding that complete suite of control.”

“The more that we understand what happens in hibernation, the better we’ll be able to mimic some of these phases.”

Drew has published research showing that adenosine receptors in the brain have to be stimulated for Arctic ground squirrels to reach the low body temperatures of hibernation. But Drew and her colleagues are uncertain about the natural molecular triggers, and how to replicate the effect using drugs. “The more that we understand what happens in hibernation, the better we’ll be able to mimic some of these phases,” she says.

Meanwhile Matteo Cerri, assistant professor of physiology at the University of Bologna, who is also collaborating with ESA, is working on inducing torpor in pigs by inhabiting the area in the hypothalamus that controls energy levels. The trial is ongoing, but Cerri tells Quartz he hopes to have concrete results to discuss by summer—and certainly within the year.

Although other scientists are doubtful that humans will ever be able to survive hypothermia and hibernate for extended period of time, Cerri is confident that month-long hibernations will be feasible. “An educated guess would be 40 or 50 years,” he says “But within 100 years I would definitely say yes.”

Inducing hypothermia in humans alone is a complicated task, and long-term hibernation is far more complex. But with the interest and funding behind this research, it’s possible that such farfetched-sounding hopes will become a reality.