Sixty-five feet below the streets of Manhattan, workers are digging the city’s first new subway line since the 1940s. The Second Avenue subway, to be named the T line, will eventually stretch from 125th Street in East Harlem to Hanover Square in the financial district. The first stretch of the line, from 96th Street to 63rd Street, is set to open in December 2016, carrying more than 200,000 passengers every day.

Scientific American visited the base of operations for the dig this past April, as engineers completed the downtown tunnel and set to work on the uptown side. A 700-foot-long tunnel-boring machine, or TBM, does the actual digging, moving at a rate of up to 100 feet a day through the city’s bedrock, a blend of granite, mica, gneiss and garnet known as Manhattan schist. “The rock in the tunnel is twice as strong as concrete, and still the TBM cuts through it like a piece of cake,” says project manager Alaeden Jlelaty of Swedish construction firm Skanska. The TBM, nicknamed “Adi” for the granddaughter of an MTA official, delivers 2.99 million pounds of thrust, the equivalent of 12 Boeing 747s.

Little of the rock the machine shatters goes to waste. Each day trucks deliver debris from the tunnel to construction sites around the city, where it is used for landscaping and land reclamation.