More than 90 percent of the Earth's habitat lies in the deep sea. More than a thousand meters below sea level there is no light, but enormous pressure and freezing temperatures. The film accompanies one of the largest deep-sea research projects to date, called ARGO. Researchers around the globe are working together to measure the oceans. Each dive by their highly specialized robots reveals wondrous unknown species adapted to the conditions of extreme depth. Researchers suspect about 10 million species in the deep sea - compared to about 1.4 million on land. It is a pure animal world, because plants cannot exist without light.