I think I remember reading this novel or novella sometime during my youth (~1999-2004) in German about a boy who has six fingers.
He accompanies a male older relative (possibly an uncle or his father) to an expedition to central/south america. There he finds an indentation inside a cave that fits his hand perfectly. Putting his hand in the indentation flips the stone over on which he was standing and he falls into a body of water below. From there he finds another exit from the cave but discovers that there are no signs of modern life in the jungle around him. Instead he finds/ is found by a society heavily inspired by the cultures of central/ south america.
I don't remember much of the main story from this point forward, just gut feelings that it had typical coming-of-age topics such as bravery & peer pressure, first romantic interests, first contact to some sort of distilled spirit, tradition vs "the youth", and the mesoamerican ballgame.
The story concludes with him returning to the cave, finding the original entrance and being reunited with his uncle/ father. Subsequent trips to the cave show no signs of the indentation or possible other exits.
I vaguely recall that the body of water that he falls into in the beginning is alluded to have a "memory" of sorts.
I tried googling for it but wasn't able to find the correct keywords. I'm also not 100% certain on the details, it could have been 4 fingers instead of 6, it could have been 6 toes instead of fingers etc.
Any help would be greatly appreciated! Thank you very much for your time!