Earth
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Earth is the alledged destination of the mythical 13th tribe of Kobol, and the intended destination of the survivors from the 12 Colonies.
Commander Adama claims to know the location, but it is revealed that he's only saying this to keep morale high (Mini-Series, Kobol's Last Gleaming, Part I). President Roslin knows this and hopes to uncover the real location of Earth in the Tomb of Athena, accessible using the Arrow of Apollo (Kobol's Last Gleaming, Part I and II).