THE HOLE IN THE MAP OF THE WORLD

ON THE SURFACE, THERE'S NOTHING UNUSUAL ABOUT IT. JUST A SPOT OF OCEAN. BUT BENEATH THE WAVES LURKS SOMETHING INCREDIBLE: A MASSIVE WATERFALL. AND IN ITS MYSTERIOUS DEPTHS, THE FATE OF THE WORLD CHURNS.

- SANDRA UPSON

TIP OF ICELAND, YOU'LL FIND WHAT'S OFTEN called a "marginal" body of water. This part of the Atlantic, the Irminger Sea, is one of the stormiest places in the northern hemisphere. On Google Maps it gets three stars: "very windy," says one review. It's also where something rather strange is happening. As the rest of the planet has warmed since the 20th century-less in the tropics, more near the poles-temperatures in this patch of ocean have hardly budged. In some years they've even cooled. If you get a thrill from spooky maps, check out one that compares the average temperatures in the late 19th century with those of the 2010s. All of the planet is quilted in pink and red, the familiar colors of climate change. But in the North Atlantic, there's one freak splotch of blue.

If global warming were a blanket, the Irminger Sea and its neighboring waters are where the moths ate through. Scientists call it the warming hole.

The warming hole could be a very big problem. That's because it's a sign that something may be wrong with the Atlantic Meridional Overturning Circulation. The AMOC is the main current system that crisscrosses the ocean. It flows like a big river up, down, and across the two hemispheres. All that moving water performs an amazing service-it's basically a supremely massive, 1-petawatt heat pump for the North Atlantic.