Trans-Bay Tube BART Tunnel Oakland, California

This tunnel under San Francisco Bay carries commuters on the subway between Oakland and San Francisco, and is one of the longest immersed tunnels 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 57 tube segments. It was completed at a cost of $180 million in 1970. On the Oakland side, BART trains can be seen plunging into the ground at the base of the 7th street terminal at the Port.