Two identical Tunnel Machines (TBMs), specifically designed for the Athenian sub-soil, bore the tunnels for the two underground lines of Athens Metro. The TBMs excavate at a depth of 6 to 28 meters below the surface, well below the level of archaeological resources and public utilities, thus avoiding costly and time consuming disruptions.

The TBMs are computer operated, electrically powered and hydraulically driven. A full-face cutter head fitted with excavating bits and disc rollers breaks up rock and soil and removes it with conveyor belts. The debris is automatically loaded for disposal in dump car trains. The TBMs work their way forward 10 meters per day, o average, leaving behind a near-finished tunnel 8.48 meters internal diameter with a precast concrete lining.