This building used the Pacific Ocean to fill the world's largest indoor swimming pools. In 1896, Adolph Sutro opened the Sutro Baths, seven pools beneath three acres of glass, once the largest indoor swimming facility in the world. Instead of mechanical pumps, the building relied on the natural rise and fall of the tide to move nearly 2 million gallons of seawater through the pools every hour, powered by gravity alone. A turbine pump only stepped in when the tide dropped too low. More than a century later, engineers still design around the same idea: work with natural systems rather than fight them.

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