unlined galvanized iron pipe

unlined hole

unlined landfill

unlined tunnel

unlined underground cavern

a group of headrace tunnels

A group of multi-arch tunnels

Access tunnels

adjacent multiple tunnels

Adjacent tunnels

Bored tunnels

Boundary between deep and shallow tunnels

Cable Tunnels

Circular tunnels

Classification of tunnels

Closely adjacent tunnels

Closely Spaced Tunnels

Closely-spaced tunnels

closely-spaced twin tunnels