In the pitch-dark gloom beneath the South Atlantic Ocean off the coast of Argentina, a magical wonderland lies hidden.
It's the deep-sea domain of scuttling crabs and spindly sea stars, of ethereal jellyfish delicately rippling in ocean currents, of elusive fish and curious octopuses, and above all, of coral.
In particular, a rarely seen, cold-water coral called Bathelia candida, whose sprawling reefs create havens for entire ecosystems in the darkness.
This newly discovered landscape contains the largest Bathelia candida reef ever found, covering an area of around 0.4 square kilometers (0.15 square miles) – almost the size of Vatican City.
"I would say that it is one of the most vibrant and lush environments in the deep sea that I've ever seen," deep-sea biologist Santiago Herrera of Lehigh University in the US told Mongabay.
"When we go down into the deep, we know that food becomes increasingly scarce, and so that means that life becomes increasingly scarce. So it's a big surprise when you suddenly find large amounts of animals all together, clearly interacting in a very vibrant and dynamic ecosystem."
The floors of Earth's oceans are famed for how little we know about them.
Sunlight can only penetrate so far, its photons scattered and absorbed as they move through the medium.
Below a certain depth is permanent darkness and permanent cold, waters unlit and unwarmed by the Sun, and crushing pressures created by the weight of all the water above.
It's only in the last few decades, with the aid of technology, that scientists can peer into the inky darkness and see what dwells therein.
What emerges is a surprisingly thriving biosphere.
Photosynthesis, on which most terrestrial food webs rely, is not available in environments so isolated from the Sun; but there are oases around which life clusters, some of which rely instead on chemosynthesis – harnessing chemical energy for survival.
Some of these oases are hydrothermal vents, where heat and chemicals enter the water through volcanic cracks in the seabed. And some are cold seeps, where methane and other chemicals leak into the ocean from reservoirs under the seafloor.
Researchers aboard Schmidt Ocean Institute's research vessel (RV) Falkor (too) were searching for cold seeps using the remotely operated vehicle (ROV) SuBastian when they discovered the magnificent coral kingdom.
"We were not expecting to see this level of biodiversity in the Argentine deep sea, and are so excited to see it teeming with life," says marine scientist María Emilia Bravo of the University of Buenos Aires and Argentina's National Scientific and Technical Research Council (CONICET).
"Seeing all the biodiversity, ecosystem functions, and connectivity unfolding together was incredible. We opened a window into our country's biodiversity only to find there are so many more windows left to be opened."
Corals that live in shallower waters form symbiotic relationships with photosynthetic algae, on which they rely for energy.
Scientists have known that deep-sea corals exist for more than 250 years, but for much of that time, these species were thought of as isolated oddities grimly eking out an existence very different from their light-loving relatives.
Only in recent decades have scientists discovered that some species can build vast reefs in the permanent darkness of the deep ocean, feasting directly on marine snow and other morsels snagged from the water column, organic matter that rains down from all the life teeming above.
And these coral reefs attract other life – a third kind of deep-sea oasis.
The species documented at the newly discovered Bathelia candida reef showed a thriving ecosystem.
Basket stars, with their multi-fronded arms, were filmed feeding on marine snow in the same way as the coral. Crabs, octopuses, and fish were also partaking of the bounty. The reef lies below one of Argentina's productive fishing regions, which may help fuel the abundance of life in the depths.
Fascinatingly, this reef was not far from an active cold seep, hosting its own ecosystem, with a large patch of chemosynthetic clams.
The relationship between cold seeps and deep-sea coral reefs, if there is one, is not well understood, so it's unclear whether the two habitats are related.
Also documented during the expedition, which surveyed a stretch of ocean floor some 900 kilometers (560 miles) long, the researchers found yet another oasis: a whale fall, the carcass of one of the largest animals in the ocean, which can sustain feasting animals and microbes for years.
It's the first deep-water whale fall ever recorded in Argentine waters.
It lies at a depth of 3,890 meters, consisting of little more than bones that now serve as a habitat for other organisms. That means it may have been there for decades.
The expedition also captured footage of the giant phantom jellyfish (Stygiomedusa gigantea), one of the largest and most seldom observed jellyfish on Earth, adding yet another unusual inhabitant to the deep-sea menagerie.
These enigmatic creatures can grow to more than 10 meters long, but their deep-sea habitat makes them a rare sight – fewer than 120 sightings have been recorded.
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These wonders are likely only the beginning of the story.
Among the expedition's haul were 28 species researchers suspect may be new to science, collected from a slew of coral beds across the survey's span.
These specimens are still being studied, which means that the strange world deep below the South Atlantic has many marvels yet to reveal.