Burrow
By defintion, a tunnel is underground.
There is no underground tunnel.
It's called a FORTRESS.
it is an adjective because it is describing something.ex. the tunnel is underground-it describes where the tunnel is :)
tunnel, cave, catacomb, burrow, shaft, drift, adit, chunnel
Sea
The one word synonym for "underground passageway" is "tunnel".
oasis
the underground tunnel in goldenrod
a tunnel
underground tunnel
From the underground tunnel.