Church Fenton Tunnels?
The tunnels we used to explore at CF were in fact the tunnels used for the hot water pipes if I remember correctly. You could get to almost all the buildings (that were fenced off or boarded up) through them. Sim building, Naafi, Mess, accom blocks were all regular routes. Of course this was always done post bar closing which added to the excitement. BlairWitching as it was known to the 'younger' generation.