tunnel

A tunnel is a man-made passageway dug through the ground or through a hill or mountain. It can include subway tunnels and pedestrian underpasses as well as highway and railway tunnels.

A single tunnel can contain multiple parallel tubes (usually connected at various points). However, consecutive tubes along a single path, which are interrupted by outdoor spaces, must be listed as separate tunnels.

Passageways through buildings and other structures do not count as tunnels, but rather as parts of those structures.