For centuries, biologists took a single truth for granted: that all life ultimately runs on sunlight. Plants capture it; animals eat the plants, or eat the animals that ate the plants. Trace any food web backward and you arrive at the sun. Then, in 1977, a submersible descended to a volcanic rift on the Pacific floor, two miles down, where no sunlight had ever reached. The crew expected barren rock. They found gardens—tube worms taller than a person, clams the size of dinner plates, crabs picking through the crowd. The energy sustaining this abundance came not from the sun but from the vents themselves, whose mineral-laden water fed bacteria able to build sugars from hydrogen sulfide. The discovery did not merely add a species to the catalog. It overturned the assumption that sunlight was life's only currency, and forced biologists to imagine ecosystems—on distant moons, perhaps—that no star need ever touch.