an artificial underground passage, especially one built through a hill or under a building, road, or river

a road tunnel through the Pyrenees

the Mersey tunnel

the tunnel mouth

a long, half-cylindrical enclosure used to protect plants, made of clear plastic stretched over hoops

cover plants in rows with a cloche tunnel

dig or force a passage underground or through something

he tunnelled under the fence

the insect tunnels its way out of the plant

(of a particle) pass through a potential barrier