Scientists say they have discovered the world's largest coral — measuring longer than a blue whale — on an expedition in Solomon Islands.
A crew of National Geographic's Pristine Seas marine scientists found the massive standalone coral while surveying the country's ocean health.
The coral measures 34 metres wide, 32 metres in diameter and 5.5 metres high, which is about the size of five tennis courts.
It eclipses other corals previously thought to be the world's largest, including one in American Samoa known as "Big Momma", the expedition's scientists said.
Team members spotted it in the Three Sisters island group in the Makira-Ulawa Province of Solomon Islands, and first thought it could be the remains of a shipwreck.
Diving below, they found a giant coral colony they estimate has grown for 300 to 500 years.
National Geographic explorer in residence Enric Sala said it was a significant scientific finding that showed why the world should protect more oceans.
It was "like finding the world's tallest tree", he said.
"At a time where we can observe every square inch of the land with satellites and drones, the ocean below the surface continues to hold mysteries like this one," he said.
'A beacon of hope'
Lead scientist on the Pristine Seas expedition Molly Timmers encountered Big Momma in American Samoa 20 years ago.
"While Big Momma looked like a huge scoop of ice cream plopped down on the reef, this newly discovered coral is as if the ice cream started to melt, spreading forever along the seafloor," she said.
The coral discovered in Solomon Islands, of the Pavona clavus species, has a rippling shape that mirrors the ocean's surface.
It's mostly brown with patches of bright yellow, blue and red. And the coral appears healthy, expedition scientists said.
It differs from a coral reef, which is a network of many coral colonies.
The National Geographic scientists estimate the giant coral in Solomon Islands is made up of one billion coral polyps — the small marine animals that form corals and build reefs.
Coral scientist for the expedition Eric Brown said large corals have high potential to reproduce, giving them a major role in helping coral reefs recover.
"While the nearby shallow reefs were degraded due to warmer seas, witnessing this large healthy coral oasis in slightly deeper waters is a beacon of hope," he said.
An underwater 'Opera House'
Adam Smith, an adjunct associate professor with James Cook University who was not involved in the expedition, said pavona clavus was a "giant" of the ocean that could live for hundreds of years.
He said the coral discovered in Solomon Islands would be a "cornerstone" for its local environment.
"It's a bit like the Sydney Harbour Bridge, or the Opera House, to Sydney. A coral such as this would be really notable for that area and all the marine life in that area," said Dr Smith, who is also chief executive of consultancy Reef Ecologic.
University of Queensland marine scientist Peter Mumby, who also was not involved in the expedition, called the newly-discovered coral "astonishingly large".
"That type of coral … do tend to grow very large. And they provide a very valuable habitat for small fish, and that often attracts cleaning fish, which then attracts big animals like manta rays to come in and [get cleaned]," he said.
"So they are really important parts of the reef."
Professor Mumby said one reason for the coral's large size could be its location away from the Pacific's main cyclone belt.
"I would suspect in this case that this coral has existed in a very benign environment that's sheltered from major disturbances," he said.
"If it's in the Solomon Islands, it won't be subjected to very frequent major storms."
Mr Brown said most giant corals have been found in select areas like the Great Barrier Reef, the central part of the South Pacific, and now in Solomon Islands.
"There seems to be something going on in this particular area of the world that has a sweet spot for these giants," he said.
Old, but not immortal
Solomon Islands is already renowned for its diverse marine environment.
It has hundreds of species of coral and is also home to one of the region's most active underwater volcanoes, Kavachi.
But the country's latest marine discovery is "monumental", Solomon Islands Ministry of Fisheries and Marine Resources officer Ronnie Posala said.
"It reinforces the importance of our ocean, which sustains our communities, traditions, and future," he said.
"Such discoveries remind us of our duty to safeguard these natural wonders, not only for their ecological value but for the livelihoods and cultural identity they provide."
Chief Dennis Marita, a member of the local community where the coral was found and director of culture at Solomon Islands' Ministry of Culture and Tourism, expects the discovery will attract tourists and more scientists.
"The island is an unknown part of the world. Not a lot of people go there, just a few locals, and very much those people staying around in nearby islands," he said.
"Suddenly people will start coming to the island, but then we need to be prepared for them. And also we need to ensure that the coral is safeguarded."
Scientists embarked on the months-long Pristine Seas expedition to Solomon Islands in October to study its marine environments, with the aim of helping the country fulfil its commitments to protect its oceans.
They use scuba dive surveys, deep sea cameras, environmental DNA sampling and technology including a custom-designed submersible.
Solomon Islands Prime Minister Jeremiah Manele said the country's survival depended on healthy coral reefs.
"This exciting discovery underlines the importance of protecting and sustaining them for future generations," he said.
Marine experts warn that even corals of the size discovered at Three Sisters islands are vulnerable to higher sea temperatures caused by climate change.
"The hope would be that even if it does experience a bleaching event, its huge size means that maybe some of it would survive and then help the rest of it recover," Professor Mumby said.
"But in general, we can't take these sorts of corals for granted, even though they've been with us for many, many years."
