This week TBM Mom Chung started digging, kicking off construction of San Francisco's first new subway tunnel in decades. Over the next 10 months, the 350-foot-long, 750-ton machine will excavate and construct the tunnel that southbound T Third Line trains will use when the Central Subway opens in 2019.

Mom Chung and Big Alma will excavate and construct the 1.5-mile-long tunnels at a pace of approximately 40 feet per day, though their pace will vary based on ground conditions and other factors. Most of their journey will be through two major ground formations: the Franciscan complex, a bedrock formation that forms Nob Hill; and the Colma formation, a dense mixture of sand and clay.