An underwater coral skyscraper taller than the Auckland Sky Tower has been discovered in the Great Barrier Reef.
Scientists from Australia's James Cook University said they were "surprised and elated" to find the 500m structure, the first detached coral reef to be discovered in 120 years.
The team aboard the Schmidt Ocean Institute's Falkor ship made the discovery last week as part of efforts to map the reef sea floor.
On Monday the Institute, a non-profit founded by former Google chief executive Eric Schmidt and his wife Wendy, sent its underwater robot SuBastian to explore the reef, which is just under 1.6km at its base.
SuBastian, equipped with robotic arms and a camera, took coral from the reef for scientific inspection, livestreaming its exploration on YouTube.
Taller than London's Shard (310m) and the Sky Tower (328m), the structure stretches from the sea floor to reach its highest point 40m below the surface.
Starting at the base, the robot climbed to the apex of the coral mountain, capturing colourful colonies, unusual shells, shoals of fish and sharks on camera.
It is one of a series of robotic dives planned by the institute to explore the oceans around Australia.
In April its scientists discovered what is believed to be the longest-ever recorded sea creature, a 45m siphonophore.
The detached reef is one of several near the Cape York Peninsula in northern Australia which have been discovered since the late 19th century.
Dr Robin Beaman, who led the team, said: "To not only 3D map the reef in detail, but also visually see this discovery with SuBastian is incredible."
The team is expected to continue scoping the area until November 17, with the resulting data set to be shared on AusSeabed, which allows scientists to share their mapping discoveries.
Wendy Schmidt, co-founder of the Schmidt Ocean Institute, said: "This unexpected discovery affirms that we continue to find unknown structures and new species in our ocean.
"The state of our knowledge about what's in the ocean has long been so limited. Thanks to new technologies that work as our eyes, ears and hands in the deep ocean, we have the capacity to explore like never before."
Dr Jyotika Virmani, executive director of Schmidt Ocean Institute, said: "To find a new half-a-kilometre-tall reef in the offshore Cape York area of the well-recognised Great Barrier Reef shows how mysterious the world is just beyond our coastline.
"This powerful combination of mapping data and underwater imagery will be used to understand this new reef and its role within the incredible Great Barrier Reef World Heritage Area."