A team of scientists have discovered a massive coral reef more than 30 metres below the water's surface off the coast of Tahiti.

The deep sea reef is in pristine condition, with lower water temperatures protecting it from coral bleaching events that have damaged nearby reefs.

"It was magical to witness giant, beautiful rose corals, which stretch for as far as the eye can see. It was like a work of art," said French photographer Alexis Rosenfeld, who was among the team that made the discovery.

At three kilometres long, UNESCO says the reef is one of the largest in the world. The discovery suggests there may be many more undiscovered reefs in our ocean, with only 20 per cent of the seabed mapped by scientists to date.

"We know more about the surface of the moon or the surface of Mars than the deeper part of the ocean," UNESCO's head of marine policy, Julian Barbiere, told Reuters.

"It also raises questions about how coral reefs become more resilient to climate change."

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