Rennes-le-Château is a large, floating landmass that appears in Xenosaga Episode III, which Shion and her allies are asked to investigate at the behest of Juli Mizrahi. Resembling a valley of mountains and forests of approximately seventy kilometers in diameter, with a hemispherical bottom half, it appears as if it has been listerally "scooped out" of a planet. Margulis claims that the mass is actually a portion of Lost Jerusalem, or Earth, that somehow appeared in space. Despite its abnormal location, there is consistent 1G gravity, breathable air on the surface, as well as a set of ancient ruins. Deeper underground lies a labyrinth of caves, stone passageways, an elaborate ancient temple, and abundant Gnosis. In the depths of this subterranean temple is a cross and burial site which looks exactly like the resting place of KOS-MOS's consciousness that appeared in the Encephalon dive from Xenosaga Episode I.
The name is derived from a castle in France, rumored to be the center of numerous conspiracies including the lineage of Mary Magdalene and Jesus Christ.