In the crushing, cold depths of the oceans, something unimaginably huge flows inexorably, barely a few centimeters per second, along a path it has traveled for millennia. Dense, dark rivers of water toil ceaselessly around the world, making up around 40 percent of the total volume of the deep oceans. They are gigantic conveyor belts transporting heat, oxygen, carbon, and nutrients around the planet, and shaping climate and weather at a global, regional, and local scale.