What is Tunnel ?
Tunnel is (noun) a long passage under the ground The Channel Tunnel links Britain to France. The road round Lake Lucerne goes through six tunnels. They are digging a new tunnel for the underground railway. Taking the tunnel through the Alps is quicker than driving up the roads over the mountains. (verb) to dig a long passage underground They decided to tunnel under the hill rather than build the road round it. Synonym excavate (NOTE: tunnelling tunnelled. The US spelling is tunneling tunneled.)
source: Easier English, Student Dictionary Upper Intermediate Level