By Alex Kirby
BBC News Online environment correspondent
The world's oceans contain far more cold-water coral reefs than experts had realised, the United Nations says.
Bottom trawling is a major concern: Before (top) and after (bottom) the trawlers have passed through
The reefs are usually found in deeper, cooler water than tropical corals, and many are up to eight millennia in age.
A report issued by the UN Environment Programme to mark World Environment Day on 5 June says the reefs are widespread from Greenland to sub-Antarctic waters.
Some are known to harbour organisms which scientists thought had become extinct several million years ago.
The report, Cold-water Coral Reefs: Out Of Sight - No Longer Out Of Mind, is the work of the programme's World Conservation Monitoring Centre, Unep-WCMC.
Slow and deep
The international scientific team which compiled it was led by Professor Andre Freiwald, of the University of Erlangen-Nuremberg, Germany.
He said: "We are finding not only new species of corals and cold-water corals in new locations but associated organisms, like snails and clams, that were believed by palaeontologists to have become extinct two million years ago.
"That was a real surprise, and we expect many of these surprises in the future as we undertake more scientific missions."
Unep's executive director, Dr Klaus Toepfer, said: "Arguably the biggest threat to both cold- and warm-water corals is from unsustainable fishing.
"So it is incumbent upon us not only to manage deep-sea fisheries better, but all fisheries, so that there is less pressure on the deep and the shallow parts of the seas."
Many of the fish living around cold-water corals are slower-growing and have lower reproductive rates than species like herring and cod which live in shallower waters.
Fishing fleets are increasingly targeting some of the deep-water species, like the orange roughy, roundnose grenadier and black scabbardfish.
In the viewfinder
Other threats they face include oil and gas exploration and production, waste disposal, and cable-laying.
Until recently scientists had thought cold-water corals were largely confined to temperate waters in the Northern Hemisphere. But new surveys have found them off the Galapagos Islands, Brazil, Angola and Indonesia.
Typically, they live in cool water along the edges of continental shelves, in fjords, and around offshore submarine banks, vents and seamounts.
At these depths, everything grows very slowly
They are generally found in waters from 4-13C at depths between 200 and 1,000m. But they can survive at just 40m, or as deep as 6,300m.
Unlike tropical corals, they do not have communities of algae living with them. They feed on plankton and other organic matter.
Their extent and complexity are gradually coming to light through the development of more sophisticated underwater cameras and deep-sea vehicles.
The known locations of cold-water corals