the way I undersatnd it the cript is what's below that slab (seeing how this place was just a dark empty room). I think this is some sort of a bug and you should try reloading from a recent game.
I might be wrong though, I had this very same bug but since there is no way to die in the game I haven't saved since about the time I discovered the underground tunels (yo know where the geologist died...) so I was to lazy to do it all again.
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