Beyond the reach of the last tendrils of sunlight, far beneath the waves, lies the planet’s largest — and strangest — habitat. Although long thought too extreme for any form of life, a decade-long exploration has revealed a startling range of exotic new species and alien ways to eke out a living in the perpetual darkness.

Sea angels, jewel squid, helmet jellies and a 2m-wide octopod that flies with ear-like fins are among more than 17,000 new species discovered during 210 expeditions undertaken to explore the deep ocean for the international Census of Marine Life.

More than 300 scientists from 34 nations have crammed themselves into deep-diving submersibles or piloted robots from research vessels on storm-racked seas far above, while automated drones have weaved through undersea chasms never seen by Man. Sediment cores, trawls and dredges have scoured for clues about the nature of the deep.

An expedition to the mid-Atlantic ridge this year by Russia, Brazil, South Africa and Uruguay, discovered what is thought to be a new species related to the octopus, nicknamed the “Jumbo Dumbo” for its passing resemblance to the fictional flying elephant.

“If it came up in a trawl it would just be a lump of jelly, but photograph it from a submersible, and it’s very beautiful and graceful,” said Odd Aksel Bergstad of the University of Bergen, the leader of that cruise. “We know very little about how they live. They’re predators but we don’t know what they feed on or how they reproduce. At least one of the nine kinds we found is probably a new species.

“Because it provides an oasis of topographical relief in the centre of the ocean, we found a high concentration of animals on the Mid-Atlantic Ridge.”

Much life in the deep relies on death in the sunlit waters above. While most food comes from the falling remains of tiny marine organisms, occasionally the biggest animals on the planet crash to the seabed. Seventeen species of “zombie bone-eating worms” — otherwise known as Osedax — survive on the rare bounty of a sunken whale.

In the deep, unidentified species are often the norm, not the exception. One cruise yielded 680 specimens of fly-like copepod, only seven of which could be identified.

“The abyssal fauna is so rich in species diversity and so poorly described that collecting a known species is an anomaly,” said Dr David Billett of the National Oceanography Centre, Southampton. “Describing for the first time all the different species in any coffee cup-sized sample of deep-sea sediment is a daunting challenge.”

“New species aren’t news for deep- ocean scientists, they’re a problem,” agreed Dr Robert Carney of Louisiana State University, one of the leaders of the census. “The figure of 17,000 species is just what’s made the logbooks, it’s what we can deal with. If you want the real figure you can multiply that by a hundred or a thousand.”

The eternal darkness has forced life to find sources of energy other than sunlight. Discovered in the 1970s, deep-sea hydrothermal vents use microbial life to help them to feed from chemicals in the scalding hot water. Less well known is a tubeworm known as Lamellibrachia, found in the Gulf of Mexico, that lives by prospecting for oil.

Like a miniature drill rig, the tubeworm has a short section protruding from the seabed but a long, fragile tube beneath that it uses to probe for petroleum, allowing it to thrive anywhere below 500 metres deep.

Scientists say that study of life within the seabed is vital for determining the viability of schemes to combat climate change by fertilising areas of the ocean to encourage the growth of carbon-consuming microscopic plants.

The census divided the survey into five separate zones: the continental margins, where the shallow shelves fall away to the deep ocean; the mid-Atlantic ridge, a section of the oceanic mountain range that snakes through all oceans but the Arctic; the abyssal plains that separate the two; seamounts, lone underwater mountains and volcanoes thought to number 100,000 but of which only 100 have been sampled in detail; and finally the specialised communities of hydrothermal vents and cold seeps.

Only the “hadal depths”, ocean trenches that plunge as deep as Everest is high, remained out of reach of detailed survey.

Information from the census will be used to inform efforts to protect the diversity and abundance of deep-sea species. Fishing the depths relies on bottom-trawling that can destroy fragile habitats before their existence is even realised. The offshore oil and gas industry is drilling in ever deeper water, and plans to mine rich mineral deposits on the seafloor are in prospect.

Dr Carney worries most about the prospect of the deep sea being used as a dumping ground, despite a current ban. “The question of what we are going to do with all our high-level radioactive waste is unanswered,” he said. “Ignorance is our main enemy. Before anyone starts to consider the deep ocean as a wasteland, we need to know what’s there.”

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