Many of them. It's how the characters and workers move around unimpeded by the crowds.
By defintion, a tunnel is underground.
There is no underground tunnel.
it is an adjective because it is describing something.ex. the tunnel is underground-it describes where the tunnel is :)
oasis
Sea
The one word synonym for "underground passageway" is "tunnel".
Burrow
underground tunnel
Burrow
From the underground tunnel.
a tunnel
the underground tunnel in goldenrod