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This tunnel under San Francisco Bay carries commuters on the subway between Oakland and San Francisco, and is the longest immersed tunnel in the world. Immersed tunnels are constructed using pre-made tubes that are floated to the location and sunk into place. The segments are then attached under water. This tunnel, built by the Bechtel Corporation from 1966 to 1969, is nearly four miles in length, and is composed of over 50 tube segments.
