The world’s largest circulation, reaching into the ocean’s depths, is due to the density gradient between dense water sinking and then gradually rising to the surface. Dense water is produced when large amounts of sea ice forms in the Southern Ocean and the Sea of Okhotsk, setting off the Thermohaline Circulation. Using satellite data, we are now able to estimate sea ice production, allowing us to identify the formation area and variability of dense water. We have also revealed that production of sinking dense water is gradually decreasing due to recent global warming.