On February 17, 1977, the research submersible Alvin descended about 2,500 meters to the Galápagos Rift in the eastern Pacific. The geologists inside expected to confirm warm water rising through cracks in young volcanic rock. Instead, the lights revealed clusters of white clams as long as a ruler, brown mussels, pale crabs and other animals crowded around shimmering water.

Later dives found fields of white tube worms with red tips, standing like flowers in the darkness. Taken together, the observations revealed the first large animal ecosystem known to have a food web based on chemical energy from inside Earth rather than sunlight.

The discovery is often compressed into one dramatic last-minute sighting. The historical record shows a sequence: a towed camera first photographed the clam bed, the first Alvin dive confirmed an active vent and its animals, and subsequent dives documented the tube-worm communities.

Thirteen photographs changed the dive plan

The expedition did not arrive at a random patch of seabed. Measurements made in 1976 had detected small temperature anomalies near the Galápagos spreading center, where the Cocos and Nazca tectonic plates pull apart and new ocean crust forms. Geologists predicted that seawater might circulate through the hot rock and return to the seafloor as a spring.

On February 15, 1977, a camera sled called ANGUS was towed above the rift. It took about 3,000 photographs during a long survey of the seafloor. Near a three-minute temperature spike, 13 frames showed hundreds of large white clams and brown mussels covering lava that looked barren immediately before and after. Woods Hole Oceanographic Institution’s history of the discovery reconstructs the photographs and dives from expedition records and participants’ accounts.

On February 17, pilot Jack Donnelly took geologists Jack Corliss and Tjeerd van Andel down in Alvin. Guided by acoustic beacons, they crossed fresh lava until they reached the clam bed. Warm water shimmered from cracks and turned cloudy blue as dissolved material precipitated into the cold ocean. The scientists measured water several degrees warmer than the surrounding bottom water and named the site Clambake 1.

The first site covered an area roughly 50 meters across. The clams reached about 30 centimeters long. Other dives during the expedition found mussels, crabs, an octopus and strange orange animals initially called dandelions. At a warmer site named the Garden of Eden, red-tipped tube worms rose from long white tubes.

These were diffuse, relatively low-temperature vents rather than the tall, superheated “black smoker” chimneys that would be documented on the East Pacific Rise in 1979. A hydrothermal vent is still an accurate term: NOAA describes the process as seawater entering fractures in ocean crust, being heated and chemically altered near magma, then returning to the seafloor.

The deep ocean was supposed to live on falling scraps

Scientists already knew the deep sea contained animals. The usual explanation for their food was material produced near the sunlit surface. Dead plankton, waste and other organic particles sink slowly through the water as “marine snow.” By the time that material reaches the abyss, much of its usable energy has been consumed.

That model suited the sparse life found across much of the deep seabed. It did not explain a dense oasis of unusually large animals around fresh volcanic cracks. The 1977 expedition had no deep-sea biologists aboard because the mission was designed to investigate geology and geochemistry. When the small supply of formaldehyde ran out, specimens were reportedly preserved in strong vodka.

The decisive clue was the smell of rotten eggs when vent-water samples were opened on the ship. The water contained hydrogen sulfide, a toxic compound that certain microbes can use as an energy source.

Chemistry replaced light at the base of the food web

Photosynthetic organisms use light energy to build organic matter from carbon dioxide. Chemosynthetic microbes can also fix carbon dioxide, but they obtain energy by driving chemical reactions. At the Galápagos vents, bacteria oxidized reduced compounds such as hydrogen sulfide carried up in hydrothermal fluid.

That microbial production supported the larger animals. Some grazed on free-living bacteria or microbial mats. Others housed bacteria inside their own bodies in a mutually beneficial partnership. NOAA’s comparison of photosynthesis and chemosynthesis explains how vent bacteria combine carbon dioxide, sulfide and oxygen to make organic carbon.

The giant tube worm Riftia pachyptila represents the most extreme version of the arrangement. An adult has no mouth, gut or anus. Instead, an internal organ called the trophosome contains billions of chemosynthetic bacteria. The worm’s red plume absorbs oxygen and sulfide from the water, and its specialized blood transports both to the bacteria. In return, the microbes make the organic molecules that feed their host. NOAA’s vent food-web guide describes this internal partnership.

Large vent clams and mussels also host chemosynthetic bacteria, commonly in their gill tissues. The animals are not eating volcanic rock. Their microbial partners are turning an energy-rich chemical imbalance into biomass, just as plants turn a light gradient into biomass at the surface.

“Without sunlight” needs one qualification

The vent discovery showed that sunlight does not have to supply the primary energy for a large ecosystem’s food production. That was the conceptual change. It did not show that every component of the Galápagos community was completely disconnected from the sunlit world.

Many of the microbes oxidizing hydrogen sulfide use dissolved oxygen. That oxygen is largely produced by photosynthesis at the surface and mixed through the ocean. The clams and tube worms are animals, so they also require oxygen. A National Academies history of the discovery makes this distinction explicit: chemosynthesis powered local food production, while the aerobic reactions still used oxygen generated by photosynthetic life.

Other microbes can perform chemosynthesis without oxygen, using compounds such as sulfate or carbon dioxide as electron acceptors. So life can obtain energy in complete darkness through several chemical pathways. For the spectacular animal communities seen in 1977, the accurate statement is that their food web was not based on sunlight, not that the entire system had no indirect connection to photosynthesis.

A new map of where ecosystems could exist

The discovery changed oceanography because it connected plate tectonics, seawater chemistry, microbiology and animal ecology in one system. Vents have since been found along spreading ridges and volcanic regions in oceans around the world. Individual fields can switch on, shut down or be buried by lava, yet specialized animals disperse between these temporary islands of chemical energy.

It also expanded the search for habitable environments beyond places reached by starlight. NASA now studies terrestrial vents as possible analogs for environments beneath the ice of ocean worlds such as Europa and Enceladus. Those moons could contain liquid water, rock and chemical gradients even though sunlight cannot reach their subsurface oceans. NASA’s astrobiology field-site program describes deep hydrothermal systems as laboratories for testing how life might function in such settings.

The 1977 team went looking for evidence that seawater circulates through newly formed ocean crust. They found it. The larger surprise was that geology was feeding biology on a scale no one had documented before. In the darkness around the vents, the base of a food web was not falling from above. Microbes were building it from carbon dioxide and chemicals rising out of the planet.