Underground caching is perfectly possible.
You could always have a "letterbox hybrid" style cache with instructions/directions on how to find the box from the entrance on the cache page (60 paces, right turn, crawl for 20 metres etc. etc.).
I'm keeping an eye out for that one!
er...check out your own Top Ten Caches Stuey, I think you've already been there!!! Nothing like a bit of self advertising []
Anyway, I once took part in an Orienteering Competition in monkton Farleigh Mine, it was the most surreal experience I've had and the first time I've used a 3D map!