3rd point of triangle…

Locke’s discovery and then re-creation of The Swan’s blast door map…

http://lostpedia.wikia.com/wiki/Blast_door_map

… this map (the LOST island itself, after all) and Blue’s “Concerto for Europe” being one and the same.

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Location of the hatches, with The Pearl being the central-most, and the location of the “?” on the blast door map.

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Julie and Locke “composing”.