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The Leviathan is an enormous serpentine creature living in the methane oceans of Titan. Records of its existence date back to the Golden Age, as its presence registered on subsurface scanning equipment. It can be seen through the windows surrounding the New Pacific Arcology's CPU core in the Festering Halls during the missions Utopia and Ace in The Hole. The Ghost mentions that the arcology's sensors can detect a large object moving under The Rig, and although the sensors can't determine what it is, this is likely the same creature. Guardians have also observed serpentine, finned creatures breaching the surface of Titan's oceans, which are likely the Leviathan or other life-forms native to Titan.
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