Tunnel-boring machines are genuine marvels of modern technology. They are self-contained contraptions big enough to build entire subway tunnels, a job that used to require thousands of pick-axes and the sacrifice of quite a few lives. But once construction is finished, getting the machines out can take months.
This timelapse shows the breakthrough and removal of two TBMs working on the San Fransisco Central Subway project, Big Alma and Mom Chong. Retrieval has to be planned before the project even starts, with a huge retrieval shaft dug years in advance. Once the TBMs reach their destination, they break through the wall, and are slowly disassembled piece-by-piece over the course of months to finally reach the surface again. [Central Subway Blog]