The Northern Hemisphere winter of 2014-15 was the warmest on record globally, according to the National Oceanic and Atmospheric Administration.
But one area of the North Atlantic was the coldest on record... shown in blue on this map.
This cold pool may be an indicator of a dramatic slowdown in the Gulf Stream.
The Gulf Stream transports vast amounts of heat north from the equator to the pole, passing off the East Coast of the U.S. and into the North Atlantic.
A slowdown like this in the current has not happened for a very long time, perhaps as long as 1,000 years.
It is most likely related to the melting of the Greenland ice sheet.
The influx of freshwater from the ice sheet is lighter and colder than heavier, salty water that typically occupies that area.
It tends to sit on top of the water, interfering with the sinking of dense, cold and salt-rich water.
The Gulf Stream transports more water than "all the world's rivers combined," according to the National Oceanic and Atmospheric Administration.
A rapid slowdown in the current would increase sea level rise rates along the Mid-Atlantic and Northeast coasts of the U.S.
It could also bring much cooler conditions to parts of northern Europe.