A tunnel is a passage dug through or under something, usually through a mountain, under a river, or beneath a city. Tunnels let people, trains, cars, or water travel through obstacles instead of having to go over or around them.

Some of the world's most impressive tunnels stretch for miles underground. The Channel Tunnel, or “Chunnel,” runs 31 miles beneath the English Channel, connecting England and France. Cities like New York and London have elaborate subway tunnels carrying millions of passengers every day through underground networks. Engineers build tunnels by drilling, blasting, or using giant boring machines that chew through rock like mechanical earthworms.

Animals dig tunnels too. Prairie dogs create entire underground cities with tunnel networks for their colonies. Rabbits tunnel beneath fields to build warrens. Even ants engineer intricate tunnel systems in the dirt.

As a verb, tunnel means to dig or make a tunnel. The word can also describe focused attention that blocks out everything else. When you have tunnel vision, you're so focused on one thing that you miss what's happening around you, like staring so hard at your phone that you don't notice your friend waving at you. This kind of narrow focus can be useful when solving a difficult problem, but it can also mean missing important details nearby.