The City of Lost Children (1995)
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While the map shows the way to safely navigate through the minefield, it also reveals the secret location of Crank's Laboratory. It is for this latter use that One and Miette need the map.
This is revealed (albeit equivocally) in the conversation between The Original (credited as "The Diver") and Miette:
The Original: "The Children!"
Miette: "What about them? What do you know?"
The Original: "He takes them. The Devil takes them away. In his boat.... he takes them to Hell"
Miette: "What Hell?"
The Original: "The tattooed man: he knows where. The map...the map of the minefield is tattooed on his body"
Miette later remembers this latter quote in flashback after receiving Uncle Irvin's "Dream" message. Edit (Coming Soon)