This might be worth giving a shot, for some of us here. It's not exactly a horror, but... Peter Cushing, Caroline Munro, and MONSTERS. Close enough, right?!? I mean, jeez Louise, what the hell do you want?!?
Peter Cushing is interesting to watch, in it. He doesn't come across as he usually does. It's almost like he was pretending to be Roddy McDowall, pretending to be the character. Now, I might watch Fright Night (1985) while this is still fresh in my mind, and see if my head explodes when I watch Roddy McDowall pretending to be Peter Cushing (and Vincent Price, at the same time), in that movie.
A Victorian scientist and his young American backer set off in their new earth-boring machine with a short test on a Welsh mountain. Unfortunately, the thing is rather more powerful than expected and they end up in an enormous cavern at the center of the earth. Here they find a race of humans enslaved to evil oversized prehistoric birds with extra-sensory abilities. The travelers decide they can be some help, the more so as at least one of the slaves is very eye-catching.