It's been 50 years since BART began shuttling passengers under the Bay between Oakland and San Francisco. And with a recent seismic retrofit of the Transbay Tube complete, the agency is celebrating the big anniversary today.

It was September 16, 1974 when the first BART riders got to experience the engineering marvel that is the Transbay Tube. Most of us take it for granted that there's an underwater tunnel that connects SF with the East Bay, even though most of us have never seen the inside or the outside of it.

The 3.6-mile-long tube lies in a specially dug trench under the Bay — 135 feet below the surface at its lowest point. And its design and construction were a true feat of 20th Century American engineering, down to its patented, earthquake-resistent joints that are designed to flex and stretch in the event of seismic shaking.

An eight-year project to retrofit the tunnel and strengthen it further against big earthquakes was completed in 2023, with the intention that the Transbay Tube can now withstand a 1,000-year earthquake event, which would be significantly larger than the Loma Prieta quake — a 6.9M — in 1989.

In the video below, you can see BART operator Donna Wilkinson being interviewed shortly after that earthquake occurred. Her train came to a stop in the middle of the tube as the quake struck, and, she says, "We didn't feel a thing." This is in part due to the fact that surface waves in earthquakes aren't felt the same way underground — or in this case underwater — and because the Transbay Tube sits on a bed of sand and gravel that experiences less shaking generally.

The project to connect the two sides of the Bay with an underground rail tunnel began its planning stages in the late 1940s. But as BART recounts in this history essay, legendary local kook Emperor Norton, visionary that he was, actually "decreed" in 1872 that an underwater rail tunnel should be built under the Bay, even planning that it should come above ground in Oakland pretty near where the actual Transbay Tube now does.

The design stage for the Tube happened in the mid-1960s, and fabrication of the Tube's 57 sections, each about 330 feet long, began in 1965. The process occurred at the Bethlehem Shipyards facility in South San Francisco, but a lot of the design and engineering work likely occurred in Bethlehem Steel's offices in Dogpatch — now home to RH's fancy's showroom and restaurant.

Each 10,000-pound segment of the tube, once constructed, was floated out into the Bay on specially designed catamaran-like barges, which then lowered each segment into place.

The tunnel was fully connected and in place in 1969, and two BART workers — one from the Oakland side and one from the San Francisco side — ceremoniously had a Champagne dinner at the spot where the final water-tight bulkhead was removed connecting the two sides — as seen in the photo below.

It took another four years before the electrical, the train control system, and the tracks were all installed in the Tube, and testing of trains began in August 1973. The first riders were able to pass through the tunnel 13 months later, in September 1974. This was two years after revenue service had begun in the East Bay, taking passengers between Fremont and MacArthur stations only.

Former actor Kenny Meyers, who ended up working for BART for 48 years, remembers his first job at BART in 1973, which was to go down into the tunnel and power-wash the insides to prepare for its public debut.

"At the time, I didn’t know anything about the project," Meyers says in an interview on BART's blog. “We were looking at each other going, what is this thing? At 20 years old, it was an eye-opening experience. That’s when I realized how crazy engineering is."

The retrofit project alone was a terrific feat of engineering, as BART explains, involving a nightly trip for workers down into the Tube on a work train that was sometimes 17 cars long, with each car carrying different piece of equipment or intended for a specific job in the tunnel.

The animation below shows what the work train might look like on a given night.

If you were ever waiting for a train after 9 pm in recent years and were told that BART was single-tracking through the Transbay Tube, this was why.

BART says that many workers worked 12-hour days, sometimes six days a week, do bring the retrofit project in on time.

"Working those long hours was a sacrifice, for sure,” says BART Structures Equipment Operator Frank Greco. "But I felt like I was a part of something bigger than myself. The Tube is such a big part of BART. It’s something thousands of people travel through on a weekly basis. Just to be a part of that, I felt very fortunate."

Next time you pass through the Tranbay Tube, make sure to remember it's now a 50-year-old, and engineers say it can live another 50 years or more, easily.