long, short
narrow, wide
dark
winding
the end of the long, winding tunnel

subterranean, underground
connecting, entrance, escape, service
hidden, secret
a vast network of secret underground tunnels

rail, railroad (AmE), railway (BrE), road, subway (AmE), tube (BrE)
sewer
wind
a wind tunnel for aerodynamic experiments

go through, use
disappear into, enter, go into
The train disappeared into a tunnel.

come out of, leave
bore, build, construct, dig, drill, excavate, make
They've built a new tunnel through the mountain.
The initial section of tunnel had to be dug by hand.

run
A service tunnel runs between the two buildings.

lead
a tunnel leading into the hill

connect
Adriana hid in the tunnel connecting the house with the stables.

open out, open up
The tunnel opened out into a large cavern.

entrance, floor, mouth, roof, wall
system
the tunnel system below the building

through a/the tunnel

a labyrinth of tunnels, a maze of tunnels, a network of tunnels, a series of tunnels, a system of tunnels
We got lost in the maze of tunnels.
The badger sett had twelve entrances to what must have been a labyrinth of tunnels.

Tunnel is used with these nouns as the subject:
mole