How was life discovered thriving where the sun never reaches?

Mostly by accident. On February 17, 1977, during geological explorations of hydrothermal vents in the eastern Pacific, two scientists, J. Corliss and J. van Andel, looked out of the submersible DSV Alvin and saw something nobody expected: dense beds of clams. A remote camera sled had stumbled on the site two days earlier, and seeing it in person confirmed the first known deep-sea community, including animals, that runs on chemosynthesis rather than sunlight. It changed what scientists thought was possible for life on Earth.

For most of history, people assumed the deep ocean was nearly empty. That changed in the 1870s, when Sir Charles Wyville Thomson and his colleagues on the Challenger expedition hauled up a huge variety of deep-sea creatures nobody had catalogued before. In 1960, the descent of Don Walsh and Jacques Piccard to the bottom of the Challenger Deep, in the Mariana Trench, which descends to nearly 11 kilometres, turned up still more evidence: a small flounder-like fish swimming away from their craft's spotlight, in a place many had assumed was lifeless.

Most ocean life depends on sunlight reaching the surface layer, the photic zone, where photosynthesis can happen; because the ocean averages about 3,688 meters deep, that sunlit layer is a tiny sliver of the total volume, even though it holds the most biodiversity. Below it, in permanent darkness, deep-sea creatures survive on whatever reaches them: slow-falling organic debris that scientists nicknamed "marine snow," or the rare windfall of a dead whale sinking to the bottom, bringing hundreds of tons of food that scavengers and specialists colonize in successive stages. One especially strange specialist, a tube worm called Osedax, is born without a sex at all: land on a whale bone and it becomes female, land on or inside a female and it becomes a tiny dwarf male, and a single female can carry more than 200 of these males inside her at once.

But hydrothermal vents go a step further, running entirely without sunlight. Seawater seeps into cracks at the boundaries between tectonic plates, gets heated by hot rock, picks up dissolved minerals and sulfides, and gushes back out, feeding bacteria that turn those chemicals directly into energy. Those bacteria support an entire food web, including giant tube worms that can grow nearly 2.5 meters tall, and more than 300 new species have been discovered at vents, in ecosystems that may be the first evidence that life on Earth can do without the Sun.