Spectacular vistas - but no sky to be seen.
Llechwedd Slate Caverns, Gwynedd.
When you enter the main workings it is pitch black, then one by one the lights come on to dramatic effect.
Corsham, Box Tunnel, Wiltshire.
The underground stone quarries and cold war facility.
Wolstyn, Poland
SS unfinished bunkers. You get to tour them on motor bikes.
Severn Tunnel,
The underground streams, shafts and other engineering works.
The Forth Railway Bridge,
Inside the structure.