"I wonder what's going on inside the cave? I really want to know." "I have no idea."
Literal
"Cave [genitive-の] inside [location-で] what [subject-が] happening [explanatory-んだろう]? Very want-to-know [feeling-なあ]" "Guess even strike-not [emphasis-よ]"
Packed with casual-monologue features. んだろう is the casual explanatory conjectural ('I wonder what...'), combining the explanatory ん (のだ) with conjectural だろう. なあ is a warm, reflective sentence-final particle expressing personal feeling or desire. 見当もつかない is an idiomatic expression: 見当 ('conjecture, estimate') + も (even) + つかない ('does not stick') = 'I can't even make a guess.'