P4X-639 was a planet in the Milky Way galaxy, which held a Stargate. It was also subjected to coronal mass emissions from the planet's sun. The world is peppered with tornado-like anomalies and geomagnetic storms. These storms are what power an Ancient time loop machine on the world.

Originally the Ancients had settled this planet as one of their bases and colony. When a great plague, perhaps the great plague, arrived they built a time machine to travel back and fix the problem. Unfortunately the device never worked; malfunctioning by creating a perpetual time loop instead of a single jump. They shut it off and the planet was engulfed in the plague, killing any un-ascended ancients that remained. (SG1: "Window of Opportunity")

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