The Margate shell grotto in England is something like that:
https://en.wikipedia.org/wiki/Shell_Grotto,_Margate
https://www.atlasobscura.com/places/margate-shell-grotto
I think the Margate shell grotto was built by an autistic guy who lived nearby, but that’s only a guess.
https://freerepublic.com/focus/chat/4009373/posts