a passageway through or under something, usually underground (especially one for trains or cars)
"the tunnel reduced congestion at that intersection"
An underground or underwater passage.
"The engineers spent years digging a tunnel to connect the two cities beneath the river."
In plain English: A tunnel is an underground passage that connects two places by going through solid ground, rock, or hills instead of around them.
"We walked through the dark tunnel to reach the other side of the mountain."
Usage: Use this word to describe any enclosed passageway beneath the earth's surface, whether it is dug by humans for transport or created naturally by animals. Avoid confusing it with "tunnels" as a verb unless you are specifically describing the act of digging through something.
force a way through
"The miners had to tunnel through the solid rock to reach the oil reservoir."
To make a tunnel through or under something; to burrow.
"The mole worked tirelessly in the dirt to tunnel beneath the garden fence and escape into its new home."
In plain English: To tunnel means to dig through something from one side to get to the other, often while hiding your path.
"The mice tunnel underneath the garden wall to find food in the cellar."
From Middle French tonnelle ("net") or tonel ("cask"), diminutive of Old French tonne ("cask"), a word of uncertain origin and affiliation. Related to Old English tunne ("tun; cask; barrel"). More at tun.