A tunnel boring machine (TBM), also known as a "mole", is a machine used to excavate tunnels. Tunnels are excavated through hard rock, wet or dry soil, or sand, each of which requires specialized technology.

A tunnel is an underground or undersea passageway. It is dug through surrounding soil, earth or rock, or laid under water, and is enclosed except for the portals, commonly at each end. A pipeline is not a tunnel, though some recent tunnels have used immersed tube construction techniques rather than traditional tunnel boring methods.