The deep ocean currents are driven by thermohaline circulation, which is movement caused by differences in the temperature and salinity content of the water. Because cold, salt-laden water is heavier than warm water, it sinks to the bottom of oceans. To replace it, warmer water fills in, and as it subsequently cools, the rotation is repeated. This constant movement of water has often been referred to as a giant global conveyor belt or pump that slowly circulates water all over the world’s oceans.

For example, the warm Gulf Stream current is heated by the Sun, “starting” in the Caribbean. It then flows north along the east coast of North America (mostly along the United States coastline) until it reaches sub-polar waters in the North Atlantic. Between Greenland and Norway, the cold Arctic winds cool the salt-laden water almost to the freezing point. Huge amounts of the now-cold, heavy salt water sink at this point to depths of around 3 to 4 miles (5 to 6.5 kilometers) and begin the next phase of the journey, traveling southwards through the Western Atlantic Basin to the Antarctic Circumpolar Current, and then into the Indian and Pacific Oceans. This trip takes many years. Off the coasts of Peru and California, for instance, upwellings often consist of ocean waters that sank to the depths centuries before.