A tunnel boring machine (TBM) is a machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata.

They can bore through hard rock, sand, and almost anything in between. Tunnel diameters can range from a metre (done with micro-TBMs) to 19 metres. Tunnels of less than a metre or so in diameter are typically done by horizontal directional drilling rather than TBMs.

Explanatory animation movie of TBM

Tunnel boring machines are used as an alternative to drilling and blasting (D&B) methods in rock and conventional 'hand mining' in soil. A TBM has the advantages of limiting the disturbance to the surrounding ground and producing a smooth tunnel wall. This significantly reduces the cost of lining the tunnel, and makes them suitable to use in heavily urbanized areas. The major disadvantage is the upfront cost. TBMs are expensive to construct, difficult to transport and require significant infrastructure. The biggest is built by Herrenknecht AG of Schwanau, Germany to dig the 57km Gotthard Base Tunnel. It has a diameter of 19 meters.