P3X-439 is a planet in the Milky Way galaxy, once home to an Ancient colony. Less than a kilometer from the Stargate, a team from Stargate Command found an enormous (and headless, due to its age) colonnade containing a Repository of knowledge at the base.
The Goa'uld Anubis sent a reconnaissance probe to the planet to scout, which discovered the secrets the planet held. SG-1, SG-3, and SG-5 arrived first, narrowly preceding the arrival of Anubis's fleet. Flanked by Death Gliders and Al'kesh, SG-1 destroyed the repository after Colonel Jack O'Neill took the drastic step of interfacing with the device. (SG1: "Lost City, Part 1")
Speculation
The monument, to some, looks oddly like an Ancient Control chair, which leads them to wonder whether this is where the first Control chairs were made (and possibly many of the things that go with them).