The stargate unearthed in Giza in 1928 was believed to be Earth's only stargate until a second Earth stargate was discovered in Antarctica, buried in a deep crevasse about 50 miles outside of McMurdo Station. Because it was the second gate to be discovered, it was referred to as the beta stargate, however evidence suggests that it was in fact the original stargate on Earth.
The beta gate is estimated to be at least 3 million years old, predating the Antarctic glacier. When it became lost within the Antarctic ice, a second stargate had been placed in Giza and was used by the Goa'uld until the ancient Egyptians buried it roughly 5000 years ago. Two Jaffa frozen in the ice suggest that the Goa'uld might have been able to temporarily open the Antarctic gate when a crevasse opened in the ice, but it could not be recovered and remained buried and unknown until an accidental power surge caused a wormhole arc, sending O'Neill and Carter to the alternate gate.
The beta gate was officially decommissioned and sealed after its recovery from Antarctica, and it was sent to Area 51. From there it was secretly moved, under classified authorization from outside the military, to an unofficial NID landing site in southern Utah, just south of the Black Mountains, about 40 km west of Parowan. Under the command of Colonel Maybourne, rogue NID teams used the beta gate as a base of operations for covert missions to steal alien technology, matching the timing of the gate activation to SGC missions in order to hide the recognizable energy spike required to force the wormhole to arc to the second gate. Following the exposure of the covert NID off-world operations, the beta gate was returned to Area 51, sealed with a permanent iris, and put under round-the-clock guard under the authority of General Hammond.
When SG-1 became trapped aboard Thor's Asgard ship, they transported the alpha gate from the SGC aboard the ship to make their escape, and Hammond ordered the beta gate to be removed from storage and set up for permanent use at the SGC as a replacement. The Antarctic beta gate became the default stargate at the SGC, with its DHD still in storage. The original Giza gate was lost at the bottom of the Pacific when Thor's ship crashed, but it was recovered by the Russians and removed to a Russian facility in Siberia, where it then became the second Earth stargate.
The beta gate remained in use at the SGC until Anubis used an off-world weapon to establish a wormhole to Earth and create an energy buildup that would cause an explosion of sufficient magnitude to destroy Earth. The beta gate was removed from the SGC, launched into space on the X-302, and sent into a hyperspace window where it detonated over 3 million miles away. The original Giza stargate, still under the control of the Russians, became the only remaining gate on Earth. Because Russia was reluctant to incur the expense and risk of running its own stargate program, arrangements were made to return the alpha stargate to the SGC in exchange for a financial settlement, the plans to the X-302 and X-303, and the promise of Russian participation in the SGC.
Cross Reference: Antarctica, Anubis's Ancient Weapon, Area 51, DHD, Harry Maybourne, NID Off-World Operation, NID Rogue Operation, Russians, Russian Stargate, Stargate, Time Travel 3000 BC, Wormhole Arc, X-302
Episode Reference: Solitudes, Touchstone, Nemesis, Small Victories, Watergate, Redemption, Frozen, Moebius
Point of Origin of the Antarctic Stargate: (unique)