The reef, which lies at depths of more than 30m (100ft) off the coast of Tahiti, French Polynesia, is thought to be one of the largest found at such depths and seems untouched by climate change or human activities.
A research mission, led by UNESCO, found the reef, which stretches for nearly three kilometres and exists at depths down to 70m (230ft). This is around the ocean's "twilight zone," where there is just enough light to sustain life, and below which the ocean transitions into a dark abyss.
The reef probably took around 25 years to grow. Some of the rose-shaped corals measure more than two metres in diameter. This is highly unusual because, up to now, the vast majority of the world’s known coral reefs sit at depths of up to 25m.
"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 led the team of international divers that made the discovery.
"For once, it's a positive story about coral reefs in the news, which is quite rare these days," Julian Barbiere, head of marine policy at UNESCO, told CNN.
Warming oceans and acidification caused by the climate crisis have led to widespread coral bleaching. Last year, scientists found the global extent of living coral has declined by half since 1950 due to climate change, overfishing, and pollution.