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.
A slowdown like this in the current has not happened for a very long time, perhaps as long as 1,000 years.
It is possibly related to the melting of the Greenland ice sheet.
The freshwater from the ice sheet is lighter than heavier, salty water that usually 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 the rate of sea level rise along the Mid-Atlantic and North-east coasts of the U.S.
This is still a matter needing more research.