The Posey and Webster Street Tubes are two parallel underwater tunnels connecting the cities of Oakland and Alameda, California, running beneath the Oakland Estuary. The Posey Tube, completed in 1928, carries one-way Oakland-bound traffic, while the Webster Street Tube, completed in 1963, carries traffic from Oakland to Alameda.
Constructed by sinking precast concrete segments to a trench in the Estuary floor and sealing them together, these immersed tubes are operated by the California Department of Transportation. The Posey Tube is the second-oldest underwater vehicular tunnel in the US and the oldest immersed tube vehicular tunnel in the world.
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