The centre tunnel passes under the ground here, and some enterprising souls, no-doubt armed with heavy drilling equipment, knocked a hole in the roof of the bunker. Keep an eye out for it - a pleasant walk through the woods could be suddenly interrupted.

This was the roof that the Optical Factory added to the tunnel - and isn't a wartime construction.

Anyway it’s a case of clambering down the wire onto a table which someone has thoughtfully put in the tunnel itself.

Time to disappear into the depths.

"Several years back (1998 possibly) this hole was covered in a big patch of concrete, and had been breached before. We wanted to break through, but someone did it for us at some point. The rebar in the concrete was cut in 2003 by Sub Brit as some of the members couldn't fit through the gap." - Andrew