The deep sea starts where the sunlight starts to fade, around 200m below the surface of the ocean. A twilight zone extends down to 1,000m, after which almost no light penetrates. The water is cold, reaching 3ºC, and contains very little oxygen. And the weight of the water above creates enormous pressures, up to 1,000 times that at the surface.
With no sunlight, plants cannot grow in the deep sea. And while animals and bacteria have been found wherever people have looked, we know very little about these dark, cold depths. More people have travelled into space than have ventured into the deep.
We do know that deep-sea habitats are as varied as those on land: vast plains, volcanoes, the longest mountain chain on Earth, deep canyons, sulphurous geysers, and of course, a huge volume of open water.
Incredible biological discoveries have been made in the last 30 years. These include entire new ecosystems such as cold-water coral reefs and vibrant communities based around chemicals pouring from the Earth's crust, as well as a whole host of alien-looking fish and other animals. Scientists now think there may be more species in the deep sea than in all the other environments on Earth combined - by some estimates, as many as 100 million species may live there.
The deep sea is beyond most people's direct experience - but you may be more familiar with some of its creatures than you think. Fishers are increasingly targeting deep-water fisheries around the world.
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