Just days after scientists documented widespread coral “bleaching” in the Great Barrier Reef, fresh surveys have revealed the true extent of the damage. Vast regions of the biodiversity hotspot, which is the largest living structure on the planet, are currently experiencing the worst mass bleaching event in their history, with only four reefs out of 520 surveyed being spared from the effects of bleaching.

“Almost without exception, every reef we flew across showed consistently high levels of bleaching, from the reef slope right up onto the top of the reef. We flew for 4,000 kilometers in the most pristine parts of the Great Barrier Reef and saw only four reefs that had no bleaching. The severity is much greater than in earlier bleaching events in 2002 or 1998,” Terry Hughes, convenor of Australia’s National Coral Bleaching Taskforce, said in a statement released Tuesday. “This has been the saddest research trip of my life.”

Coral reefs, which are delicate marine biodiversity hotspots where corals grow in symbiosis with algae, become bleached when they are stressed — due to a rise in ocean temperatures, acidity, or both. This expels the symbiotic algae living in corals’ tissues, causing them to turn completely white.

Human-induced climate change, coupled with the effects of El Nino, is now being blamed for the rise in sea temperatures that triggers coral bleaching.

The Great Barrier Reef, which is the largest living structure on the planet, consists of nearly 3,000 individual reefs and stretches over 1,400 miles along Australia’s northeast coast. The region serves as a habitat for nearly 100 species of jellyfish, 3,000 varieties of mollusks, 1,600 types of fish, over 130 varieties of sharks and rays, and more than 30 species of whales and dolphins.

In June 2015, the reef narrowly avoided being listed as “in danger” by the Unesco, after the Australian government announced a rescue plan to curb the damage.

However, last week, Australian authorities were forced to raise their coral bleaching response level to “severe” — the highest in their response plan. The move was triggered by recent underwater surveys in the Cape York region, which showed that some reefs in the region had suffered up to 50 percent mortality.

The findings have raised doubts over the efficacy of Australia’s efforts to save the reef.

“Even more concerning, we haven’t yet found the southern limit of the bleaching,” Hughes said Tuesday. “We’ll be conducting further aerial surveys this week in the central Great Barrier Reef to identify where it stops. Thankfully, the southern Reef has dodged a bullet due to cloudy weather that cooled the water temperatures down.”