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A $288 million, five-mile tunnel began carrying the Bay Area’s water supply across the bay. The water supply originates from the Hetch Hetchy Reservoir in Yosemite National Park. The Bay Area Tunnel will increase the reliability of the Bay Area’s water system, which serves approximately 2.6 million people. The tunnel will replace a pair of leak-prone pipes, built in 1925 and 1936, that sit on wooden trestles near the Dumbarton Bridge and briefly dip underwater to avoid the bay’s shipping lane.