Don't know if anyone proposed this idea, but what if the last time the island was moved it just so happened to share the same space as the Black Rock when it materialized, trapping it in the middle of the island. This could be a way to explain this mystery, plus if the captain was like WTH happened here, jotted down his location and bearing via compass in his journal, that journal washed up in another country somehow and was found later on down the line by his ancestor who used the island for the Dhama Initiative - The Hanso Line - and then Whidmore somehow caught wind and has been trying to find it ever since.

Kyle

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