BART’s Transbay Tube is a 3.6-mile-long submerged tube that rests at the bottom of the San Francisco Bay, 135 feet below the surface. Every hour, 24 BART trains with up to 28,000 people travel across the sea floor thanks to this engineering marvel, which connects San Francisco and Oakland.
It can be easy to take the Transbay Tube for granted when you’ve got places to be, but the underwater tunnel continues to be one of the world’s most significant transit structures.
In honor of the 51st anniversary of the famous tunnel’s opening this September 16th, here’s everything you never knew about the Transbay Tube.
Origins of the Transbay Tube
As BART writes, the first idea for the Transbay Tube can perhaps be attributed to Emperor Norton, an eccentric SF resident and beloved local historical figure who proclaimed himself “Emperor of the United States” and “Protector of Mexico” in 1859. He had no real power but was known for issuing humorous unofficial decrees, including a request for an underwater tunnel between SF and Oakland in 1872.
Norton’s outlandish proposition was incidentally rendered into reality in 1947, when a joint Army-Navy Commission proposed an underwater tube to mitigate traffic on the new Bay Bridge. Construction eventually began in 1965.
The process involved dredging 5.7 million cubic yards of mud, fabricating 57 330-foot-long tube segments, and lowering the segments onto the bay floor. Expert divers helped guide the segments into place, and the next four years were spent laying tracks and installing equipment. BART sent its first train through the tube in 1973, and regular passenger service began on September 16, 1974.
At the time, the Transbay Tube was the world’s longest underwater transit tunnel and the largest and deepest immersed tube. It held this record until 2010, but has since been surpassed by several immersed transit tunnels around the world.
Today, the world’s longest underwater transit tunnel is the Seikan Tunnel in Japan, whose underwater section measures 23 miles.
Design, features, and seismic safety
The Transbay Tube is about 48 feet wide with two interior tunnels for trains traveling in each direction, flanking an interior gallery used for maintenance, ventilation, and emergency escape. The tube descends as deep as 135 feet beneath the water.
It’s designed with seismic safety features in mind, allowing for 6 degrees of movement in the case of a major earthquake. More recent retrofits include a new steel liner in critical sections and upgraded water pumping systems.
51 years after its opening, the Transbay Tube continues to be one of the world’s most remarkable transit structures, shaping the future of global transportation innovation for generations.