There is a place on the osi map near the town south of Brittain where you can run around a river and run into a secret tunnel, which leads to an enclosed valley.
The tunnel looks completely hidden from the visible end (and the other end too, obviously) and it LOOKS like solid mountain, until you walk through it.
I have tried to duplicate the effect, but nothing I do seems to work, I even tried unstaticing the area, but no effect, nor does it show anything particular on Worldforge.
Would anyone have any idea how to duplicate this effect for secret tunnels on custom maps? Suggestions would be greatly appreciated.

Thanks!