Young adult novel about a boy with six fingers who discovers a portal to a maya/ aztec inspired hidden world

Question

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!

Answer 1

I reposted this on reddit and someone found it:

It's La memoria dell'acqua A.K.A Das Geheimnis der versunkenen Pyramide by Silvana Gandolfi.

edit: Summary of the book:

Nando, an eleven-year-old boy, is unexpectedly invited by his uncle, a real Lacandon from the Mexican rainforest, to his finca. It is not until the second day in Mexico that he is to find out the real reason for this trip: The uncle has discovered an underground pyramid of the ancient Maya in the rainforest. And embedded in the floor of this pyramid is a circular alabaster disc that no one can lift. The disc bears rings with mysterious signs and in the middle are two imprints of children's feet with six toes each - exactly the kind of feet Nando has. Nando can't believe it - until now everyone thought these feet were a birth defect, and now it suddenly looks as if fate has destined him for something very special. The experiment succeeds: At the same moment as Nando places the second foot in the imprint, the disc tilts and Nando falls into a cenote - one of the many entrances to the subterranean world of the Yucatan water caves. A powerful suction takes hold of him and washes him into a huge grotto, from which Nando can no longer return to the well and is thus cut off from his uncle. And here he meets the ancestors of the mysterious people behind the mists - a grandiose but also life-threatening adventure begins.