Deep in the tropical Atlantic, in a slice of ocean almost no one will ever see, scientists have found something extraordinary: 31 brand-new species, and they did it in just two weeks.

The discoveries came in international waters off the coast of Brazil, where a team on a Schmidt Ocean Institute expedition sent an underwater robot named SuBastian into the ocean's "midwater,” the vast open water between the sunlit surface and the seafloor. It doesn't get the glamour of coral reefs or the deep seafloor, but it's the largest living space on the planet.

"The largest habitat on Earth, the midwater, is filled with incredible animals we are only just starting to understand," said Dr. Karen Osborn, the expedition's chief scientist and a researcher at the Smithsonian National Museum of Natural History.

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Siphonophores, larvaceans and amphipods, oh my!

And what animals they are. Among the finds: nine new jellyfish, seven comb jellies, seven siphonophores, four larvaceans (a tadpole-like creatures that live inside houses made of mucus), a delicate gossamer worm, a crustacean called an amphipod, and two giant single-celled organisms big enough to see with the naked eye.

Some are almost too strange to picture. One newly spotted comb jelly is nearly invisible, its body glass-clear except for a deep red gut, which is a built-in cloak to hide the sometimes-glowing prey it swallows. 

A new helmet-shaped jellyfish, from the genus Bathykorus, trails four stiff tentacles and lives so deep it's only ever been seen by robots; SuBastian's pilots captured it more than half a mile down. The team also filmed a female octopus making a meal of a bright red jellyfish, and a juvenile glass squid.

Quick work identifying the new species

Identifying a new species can take decades. This team confirmed 31 in just two short weeks.

The secret was pairing high-resolution imaging with genome sequencing done right there on the ship, which is a toolkit that lets scientists study these fragile animals without destroying them. Many are so delicate they can't survive a net. The crew even imaged living cellular structures in 3D, a first for research at sea.

For Stanford University's Dr. Manu Prakash, that 3D cellular imaging is a breakthrough of its own.

"This opens a new door for researching deep-sea physiology, linking cellular architectures to organism function," he said.

The midwater helps drive ocean processes like carbon cycling, part of how the ocean stores carbon, so understanding what lives down there matters for understanding the planet as a whole.

The deep ocean, it turns out, is still keeping plenty of secrets.