The UK's largest tropical reef has been devastated in the global bleaching event now under way.
Up to 85% of the corals in the Chagos Marine Reserve of the British Indian Ocean Territory are estimated to have been damaged or killed in the event.
Scientists say the conditions there are worse than in 1998 - the last major bleaching occurrence.
The problem is caused by anomalously warm water, which prompts the coral polyps to eject their symbiotic algae.
This drains them of their colour and is fatal unless conditions are reversed in a reasonably short time.
Unfortunately for the Chagos, the water has been persistently warm for many months.
"In 1998, the temperature that killed all the corals was probably about 29.5C. Last year, in April, at the beginning of the latest bleaching event, it was 30.5C and down to 25m. And this year scientists have been out and it's the same again," said Prof Charles Sheppard, the chair of the Chagos Conservation Trust.
Prof Heather Koldewey, from the Zoological Society of London and a CCT Trustee, led the expedition. She described what she saw as shocking: "I was there two years ago and it's always an absolute joy to go diving in Chagos because you really get to see what a reef should look like - rich, living corals with abundant fish and other marine life at densities you just don't get in other places. This was very depressing."
Bleaching is happening globally on a huge scale because of the El Nino phenomenon, which sees surface water temperatures spike in many ocean regions.
Reports in the past couple of weeks have highlighted the damage to the famous reefs off Australia and the Maldives.
The big question now is how well Chagos will recover when conditions calm down, which they should do as the El Nino subsides.
The reserve successfully bounced back after 1998, principally say the scientists because it is normally such a pristine environment.
Controversially, the Chagos Archipelago has been maintained relatively population free since the late 1960s, but this has had the effect of limiting the stress factors that can weaken corals.
"This is what makes Chagos such an important reference site for corals worldwide," explained Prof Koldewey.
"This is not an oil spill, this is not coastal pollution, sewage, or overfishing or siltation. If anywhere can bounce back, it is the Chagos Archipelago, and I hold on to that positive point of view."
Prof Sheppard said the hope would be that juveniles in deeper, cooler water will come up to re-invade the reef and re-establish communities.
The Chagos Marine Reserve was established in 2010 and covers an area of 640,000 square kilometres - more than twice the area of the UK.
The zone covered by reef is estimated to be about 60,000-80,000 square km.
Biodiversity catalogued in the reserve includes - in addition to the corals - more than 1,000 species of fish; endangered green and hawksbill turtles; the world's biggest land crab, the metre-spanning coconut crab; and breeding colonies of terns and shearwaters.