The Cave

Last night I was at a rocky beach at low tide. There was a prominent sign :-
'Beware Of Strong Currents'
I stood on a rocky platform and wondered why the currents could be so strong. Then I looked behind and to the right. There was a deep cave in the cliffs.
It sloped down to an underground river far below. Geologically unlikely.
I moved closer to the cave, I could hear the river roaring, it seemed to echo inside my head.
So when the tide rose, water would pour down into the cave, together with any unfortunate paddler or swimmer.
Suitably equipped, that could be an adventure and save years of therapy delving into the subconscious.