OUTER GREAT BARRIER REEF, Australia Its not supposed to look like that, John Rumney mutters as he pulls himself out of the ocean.

Weve just had our first sighting of the Great Barrier Reef , a 1,400-mile-long behemoth of coral off the coast of Queensland, northeastern Australia, that covers a region roughly the size of Japan. Its home to a wealth of animals, including more than 1,500 species of fish , six of the worlds seven species of threatened sea turtles and at least 30 species of mammals.

Rumney, a tour operator and fisherman whos worked in the area since the 1970s, has seen the Great Barrier countless times, so many that he calls himself more a resident of the reef than a resident of Australia. Dip your face beneath the water and youre confronted by majesty: turquoise parrotfish pecking away, yellow boulders of coral, iridescent giant clams pulsing with life.

But, with every flick of your diving fins, unnatural flashes of white appear, signs of a reef in distress, physical manifestations of climate change run amok. And visitors have noticed, sparking a burgeoning trend of last-chance tourism people coming to catch a glimpse of the reef in case its their final opportunity.

The Great Barrier has been altered beyond recognition in recent years. In 2016 and again in 2017, the structure was hit by successive mass bleaching events that left large swaths of the once-colorful corals dying or dead. Nearly one-third of the reef was killed as a result of the first bleaching, and the following year served as a gut punch that further crippled one of the worlds largest living organisms.