Australia's Great Barrier Reef (GBR) is made up of almost 3,000 individual reefs, a number now one larger after the announcement of the first discovery of a new reef in 120 years. Better still, the new discovery appears to be in robust health, despite most of the GBR having been devastated by a series of mass bleaching events. 

The GBR dwarfs all other coral reefs in part because the continental shelf off Queensland is so wide. Yet even beyond the shelf seven “detached reefs” were located in the 19th century, sitting atop tall peaks that start at great depths and reach up to near the surface.

The Schmidt Ocean Institute, currently conducting a 12-month mission to explore the poorly studied waters around these reefs, made the stunning discovery of another detached reef. At its base, the reef is 1.5 kilometers (0.9 miles) long, but quite narrow. From there it rises 500 meters (1,640 feet) – taller than New York's Empire State Building – to support a small reef 300 meters (984 feet) long and only 15 meters (49 feet) wide lying just 40 meters (130 feet) below the surface.

_{Broad mapping of the new 500-meter detached reef. Schmidt Ocean Insitute}

The reef supports an abundance of coral. Although the Schmidt team has used the underwater remote-operated robot SuBastian to climb the peak they have not had a chance to sample lifeforms directly.

Nevertheless, Dr Robin Beaman of James Cook University told IFLScience the reef supports all the main ingredients of a thriving tropical reef, with “Hard corals, soft corals, fish, and sharks.” Indeed the latter is so abundant team member Mardi McNeil of Queensland University of Technology referred to a “blizzard of fish and sharks.” This is in contrast to shallow water reefs nearby in the GBR's northern zone, most of which have not recovered from the 2016 mass bleaching event, which hit the area particularly hard.

^{The reef springs into colorful life around the 1:48:00 mark.} 

This may be more than a one-off. McNeil explained to IFLScience shallow reefs are surrounded by water that is warming alarmingly. This presents a major threat, particularly at the northern end of the GBR where corals are close to their thermal limit. On the other hand, detached reefs, surrounded by cool deep oceans stirred by stronger waves, are likely to be less affected by rising temperatures, at least in the short term. A nearby detached reef is topped by Raine Island, home to the world's largest green sea turtle nesting area, whose coral also seems to be much healthier than counterparts on the continental shelf.

One theory holds that deepwater reefs could act as a sanctuary for species unable to survive hotter conditions elsewhere, from which other reefs might one day be repopulated. 

The fact something of such significance could have been off the Queensland coast for so long without anyone noticing underlies our poor knowledge of the seafloor where shipping is not threatened, Dr Beaman told IFLScience. Although nearby detached reefs were studied a few years ago using air-borne LIDAR, surrounding areas were ignored.

The discovery was made as part of a Schmidt Ocean Institute effort to change that, mapping the entire area around the northern GBR using both the research vessel Falkor and ROV SuBastian. Beaman told IFLScience that mapping is 90 percent complete, so while he doesn't rule out finding something else, the chances are not high. Meanwhile, only 20 percent of the deep ocean worldwide has been mapped with modern tools, but the Institute is participating in a project aiming to change that by 2030.

More footage from SuBastian is available on the Schmidt Ocean Institute's website and YouTube channel.