The Ohlone people made salt from the Bay long before Europeans arrived, evaporating bay water in naturally occurring tide pools. When the Gold Rush brought a massive influx of people to the Bay Area in the 1850s, salt became essential for food preservation and industrial processing. The favorable conditions of sun and wind in the South Bay, one of only two locations on the West Coast where commercial salt extraction from seawater is viable, sparked an explosion in production. Early operations were small family affairs, but over time they were absorbed by larger companies. The Oliver Salt Company operated in Mount Eden, and Leslie Salt dominated in Newark, until Cargill purchased Leslie in 1978 to become the dominant producer in the area.

Despite being an industrial landscape, the salt ponds have become a core ecosystem for San Francisco Bay wildlife. The ponds support over one million waterbirds through the year, hosting more than 75 species. During winter and spring migration, a single pond can hold 200,000 birds. Canvasbacks, Ruddy Ducks, and the threatened Snowy Plover are among the most common species. The ponds serve the Pacific Flyway as wintering and stopover habitat for ducks, gulls, and grebes. But the ecosystem carries hidden costs. Mercury from upstream sources like the Guadalupe River enters the ponds through sediment, accumulating in the food chain. Forster's terns breeding in the salt ponds carry the highest average blood mercury levels of any birds measured in the western United States, and 79 percent of their eggs are at risk of birth defects.

At the time of the Gold Rush, the marshlands around the Bay were seen as unproductive land ripe for development. That attitude drove the conversion of 80 percent of the Bay's original marshes to other uses. The salt ponds themselves replaced tidal wetlands, trading one kind of ecosystem for another. In 2003, Cargill sold the bulk of its salt ponds to the California Coastal Conservancy, the U.S. Fish and Wildlife Service, and private foundations. The resulting South Bay Salt Pond Restoration Project, negotiated with the help of Senator Dianne Feinstein, aims to restore 15,100 acres of former ponds to wetlands, a process that may take as long as 30 years. Not all ponds are being restored; Cargill has proposed developing some into housing in Redwood City, drawing criticism from environmentalists.

The salt ponds are perhaps best understood from the air, where their geometry and color reveal the logic of evaporation. Water enters the ponds at low salinity, appearing blue-green from Synechococcus algae. As it moves through a sequence of ponds, concentrating through evaporation, the water shifts to orange and then deep magenta as halobacteria and Dunaliella take over. The final ponds, where salt crystals form, appear white. This progression, visible as bands of color across the landscape, is a living chemistry lesson drawn at a scale that only makes sense from altitude. It is one of the most distinctive visual features of the San Francisco Bay, and one of the few industrial landscapes that is arguably more beautiful than the ecosystem it replaced.