Underground passages or passages through mountains that allow you to travel from one place to another while hidden or protected.
From French 'tonnel' meaning barrel, because early tunnels were barrel-shaped. The word originally described the barrel-like shape of excavated passages before it meant the passages themselves.
The word 'tunnel' comes from barrels because medieval miners created rounded passages that looked like huge wooden barrels turned sideways—a perfect example of how we describe new things using old familiar objects!
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