The two big tunnel-boring machines, which were nicknamed Mom Chung and Big Alma after two female figures in S.F. history, just completed excavating twin, 8,300-foot long tunnels beneath downtown San Francisco for the Central Subway system. They both started in 2013, and for various reasons we may never understand, one completed the underground journey in ten months, and the other did it in about seven. Big Alma made better time, as the Chron reports.

Both massive machines, which are each longer than a football field (95 meters) and weigh 750 tons, began digging down on Fourth Street beneath the freeway overpass, with Mom Chung starting in July 2013, and Big Alma starting in November. Both made it through this month, with Big Alma finishing up and arriving in Chinatown on June 11. The tunnels they dug range in depth from 40 feet to 120 feet below ground. And this very detailed video will explain to you how they both dug and built the tunnel's concrete cylinders at the same time.

Frighteningly, both machines passed just seven feet below the BART system without requiring any disruption in transit — and, thankfully, without causing some horrible surprise collapse. Below, a rendering of how the subway systems will overlap below ground.


The machines may not have caused any disruption, or noticeable vibration, for humans above ground, apart from the ongoing construction that's closed parts of Fourth and Stockton Streets, however the vibrations definitely upset an army of ground-dwelling rats, who all scattered and made far more frequent appearances above-ground in SoMa and North Beach over the last few months. This ratpocalypse may not end anytime soon, however, as construction on the system in general is expected to continue into 2019, and the little vermin will continue to run scared seeking better and safer places to nest and eat our trash.

Anyway, the tunnel-boring machines will now have to be slowly disassembled, piece by piece, and lifted out of their holes over in Chinatown by the old Pagoda Palace Theater, which is where the Central Subway terminus will be.

And as the Chron reminds us:

Mom Chung is named for Dr. Margaret Chung (1889-1959), the nation's first female Chinese American physician, who practiced in Chinatown. Big Alma was the nickname of Alma de Bretteville Spreckels (1881-1968), a wealthy San Francisco socialite who was also the model for the woman atop the Dewey Monument in Union Square.

The entire project is estimated to be costing $1.578 billion, and is so far on schedule and on budget. You can look forward, in five years, to a new Muni Metro line that looks like this and takes you from CalTrain to Chinatown in underground comfort. In the meantime, you'll still need the Dirty 30 to get up Stockton, and there will be lots of rats. Everywhere.

Here's the most recent construction update video made by the SFMTA, from March.


Rendering via the SFMTA