A tunnel boring machine is a machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata. Crossrail used eight tunnel digging machines to construct the new tunnels under London. These giant boring machines worked 24 hours a day beneath the streets of London, excavating large volumes of ground and erecting the concrete tunnel lining. The tunnel boring machines have a rotating cutter head at the front, and a series of trailers behind, housing all the mechanical and electrical equipment required for the excavation of material. How these giant machines actually work??? Watch the video and find out!