Transbay Tube, Underwater rail tunnel in San Francisco Bay, United States.
The Transbay Tube is a rail tunnel beneath San Francisco Bay in California, United States. The passage stretches roughly 3.6 miles (5.8 km) underwater and links the cities of Oakland and San Francisco at a depth reaching 135 feet (41 m) below sea level.
Engineers built the tunnel between 1965 and 1974 by assembling 57 prefabricated sections on land and lowering them into carefully dredged trenches in the bay floor. The idea of an underwater crossing between San Francisco and Oakland originally came from a proposal by Emperor Norton in the late 19th century.
The name refers to its position crossing the bay and linking two major urban areas through an underground passage. Riders experience a brief moment of total darkness during the journey as trains pass beneath the deepest point of the water and windows show only blackness.
The crossing takes about four minutes and trains run through the tunnel every few minutes during the day. Travelers should know that there is no mobile phone signal inside the tunnel and windows remain dark during the underwater portion of the ride.
The tunnel went through an extensive retrofit project between 2020 and 2024 to strengthen its structure against seismic events near the San Andreas and Hayward faults. Workers installed special expansion joints during this project and reinforced the connections between individual sections to better withstand movement during an earthquake.
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