Deep beneath the streets of San Francisco, Mom Chung is making her move. In July, the massive tunnel boring machine (TBM) started construction on the Central Subway, the city's brand-spanking new subterranean transit line (ETA 2019).

For the last five months, the hefty azz mutha has been barreling through up to 40 feet of bedrock, sand, and clay a day.

The no-effing-around gal—all 750 tons of her—will be working with fellow large-and-in-charger Big Alma to excavate mile-and-a-half long tunnels from the Caltrain station in SoMA through Union Square, and ending up in Chinatown (both of these beauts were named for famous SF women). Because they're running from 40 to 120 feet deep, folks on the sidewalk won't even be able to feel any of the action, but, from the pics and based on this awesome timelapse, there's an awful lot going on.

The Central Subway blog has a pretty fascinating rundown on how they go about their large-scale diggin'. First a spinning cutter head sprays an environmentally safe foam to soften up the soil, before "it cuts through the earth like a cheese grater."


Excess material is shipped backward on a huge conveyor belt, and curved concrete segments are then installed around the back of the perimeter—lifted up by the machines, bolted into place by crews—which provide support for the TBM to push forward again.


If you're into underground goings-ons, there are a handful of fun TBM Twitter accounts that share pics of their findings with a bit of goofy personality; in addition to Big Alma and Mom Chung, there's Cleveland's Mackenzie, Seattle's Bertha, and the lovely Lady Bird.


All images courtesy of SFMTA/Muni Central Subway