Stretching out for thousands of metres, the pristine rose-shaped corals are a sight to behold.

This is one of the rarest discoveries of its kind - an unspoilt reef, not only larger than most others but much deeper too.

Most corals are found at depths above 25m - where they get most of the sunlight they need to reproduce.

But this huge reef, found in French Polynesia by a team of UNESCO scientists, stretches from 30m down to 70m - in what's known as the ocean's "twilight zone".

It is thought that being located so deep may have protected the reef from the worst ravages of global warming.

UNESCO's Julian Barbiere told Sky News: "We have lost about half of our coral reefs in the last 70 years. But this is healthy, it's extensive, and it implies there may be other parts of the world where we have similar coral reefs at these sort of depths.

"It's really the stuff that makes you dream - makes you understand the treasure and the beauty of the marine environment."

A team of scientists located and measured the reef using state-of-the-art diving equipment.

Until recently, very few scientists have been able to investigate reefs at depths of more than 30m because of the limitations of traditional equipment.

But divers can now use advanced gear like rebreathers which employ a specialised gas mix to allow them to stay deeper for much longer periods.

After 200 hours of diving, the team measured the reef at 3km long, and up to 65m wide - with some of the individual corals measuring 2m across.

That makes it one of the largest reefs ever discovered at these depths. It also suggests more unspoilt reefs could be located around the world this far below sea level - exciting news for marine conservationists.

"Coral reefs are very important for ocean health - we know they host about 25% of all the species we find in the marine environment," Mr Barbiere said.

"Reefs are also where we can potentially find new species and new medicines.

"We are deriving more and more medicines from marine organisms, particularly to treat cancer, arthritis and infections.

"They also protect local communities from storms and tsunamis, and provide a lot of income from tourism."

Further dives are planned in the coming months to research the reef and the marine life that it is home to.

UNESCO will also work with local communities to give them the tools they need to protect it - like designating marine protected areas.

The discovery illustrates how little is still known about the underwater world.

Currently only about 20% of the world's seabed has been mapped - meaning we know more about the surface of the moon than we do about the deep ocean.

The Intergovernmental Oceanographic Committee - which is part of UNESCO - is now working with other organisations and governments to try to finish mapping the seabed by 2030.

Along the way, the hope is that more unique and precious ecosystems may well be unearthed.