an underground passage, esp one that allows people, vehicles, etc to pass have access through or under an obstruction such as a hill, river, or building

a small underground passage, often one of many forming an interconnected system, excavated by a burrowing animal, e.g. a mole

to make (one's way) by digging

He tunnelled his way under the prison walls

(often + under/through) to construct an underground passage under or through an obstruction

said of a particle: to pass through a potential energy barrier