The twilight zone. It’s not just a spooky 1960s television series. It’s what scientists call the part of the ocean between about 600 and 3000 feet below the surface. It’s deep, it’s dark (thus, the name), and it’s relatively unstudied. But it may be home to more life than the rest of the ocean, combined, and also key to the ocean’s ability to suck carbon dioxide out of the atmosphere.