Argo is a fleet of nearly 4,000 robotic floats that drift through the world's oceans, sinking to about 2,000 metres and rising again to measure the water's temperature and saltiness. At the surface each float sends its data by satellite, free for anyone to use.
A typical float drifts for most of its cycle at about 1,000 metres, then sinks to about 2,000 metres and measures the water as it rises to the surface, completing each cycle in roughly ten days. The programme began around 2000, and in November 2007 it reached its goal of 3,000 floats spread across the open ocean. Its name comes from the Argo, the ship of Jason in Greek myth, because the floats work alongside the Jason satellites that measure sea level. Newer Biogeochemical-Argo floats also carry sensors for oxygen, acidity, nutrients and chlorophyll, the green pigment of microscopic ocean plants. Before Argo, most deep-ocean measurements came from ships on a few busy routes; now scientists can follow how the whole ocean is warming.