Rain fell on Greenland's ice sheet for the first time ever known. Alarms should ring | Kim Heacox

Climate scientists believe that if Greenland continues to rapidly melt, tens of millions of people around the world could face yearly flooding and displacement by 2030

Many people believed he couldn't do it. Ski across the Greenland ice sheet, a vast, unmapped, high-elevation plateau of ice and snow? Madness.

But Fridtjof Nansen, a young Norwegian, proved them wrong. In 1888, he and his small party went light and fast, unlike two large expeditions a few years before. And unlike the others, Nansen traveled from east to west, giving himself no option of retreat to a safe base. It would be forward or die trying. He did it in seven weeks, man-hauling his supplies and ascending to 8,900ft (2,700 meters) elevation, where summertime temperatures dropped to -49F (-45C).