Most of the weird blobby creatures that star in those viral videos live in this zone. And while they’re unfamiliar and unidentifiable to most researchers, people like Haddock and Osborn can almost always instantly recognize them—if not to the species, then at least to the broad group. “They’re our familiar friends, like chickadees at a backyard feeder,” says Haddock. “You’d be surprised at the number of interesting things you can find if you jump into the water in the middle of nowhere.”

There’s Deepstaria, a jellyfish that caused a viral sensation when it was filmed floating past an oil rig. It lacks the pulsating bell and trailing tentacles of most jellyfish, and is instead a brown, amorphous sac covered in a network of veiny lines. Some thought it was a discarded whale placenta, but no, it really is a jellyfish.

Nanomia swimming through green dye.

(Jack Costello)

There are jellyfish relatives called siphonophores, which live in colonies that can grow up to 40 meters in length. The members are all clones that bud off from each other and stay connected. These individuals can perform different roles, much like the organs in your body. In one siphonophore called Nanomia bijuga, there are clones that capture food with stinging cells and others that propel the colony by jetting water. Even among the latter set, some clones specialize in thrust, while others steer.

(This specialization can become so extreme that scientists have sometimes mistaken parts of a siphonophore for an entirely new animal. That happened in 2014 when scientists described a mushroom-shaped creature called Dendrogramma. They claimed that it could represent an entirely new group of animals; others suggested that it could "completely reshape the tree of life.” Well, no. It’s a siphonophore. It’s probably a bract—a structure that doubles as a float and a protective covering for more delicate body parts.)

There are salps—barrel-shaped animals that eat by filtering particles from the water that they pump through their bodies. Like siphonophores, they can clone themselves into long, connected chains that stretch for meters, or that coil into loops like transparent cinnamon rolls. But unlike siphonophores, they don’t sting and they aren’t related to jellyfish. In fact, they’re part of a group called the tunicates, which are closely related to back-boned animals like ourselves. If you ever see a chain of salps, think of them less as aliens and more as family.

Another group of tunicates, the pyrosomes, also form large clonal, filter-feeding colonies. Theirs are shaped like socks—tubes that are sealed at one end and open at the other. Some are smaller than thimbles; others are 60-foot-long windsocks. The colony-members all face outwards: they each suck in water, sieves it for food, and expel the rest into the central part of the sock. Those inward jets also propel the colony along, albeit very slowly. A pyrosome won’t swim up to you, but some are large enough to swim into—and at least one penguin has died in this way.

In the midwater, even familiar groups of animals can have strange representatives. Osborn studies polychaete worms, the group that includes common earthworms. But her subjects have an unearthly beauty, with bright colors and undulating paddles, “Most people could stare at them for hours,” she says. “They’re really mesmerizing.” Some are literally mesmerizing: they dazzle or distract approaching predators by releasing “bombs” of glowing chemicals from their heads.