In the Ancient outpost buried beneath the ice of Antarctica, SG-1 discovered a throne-like chair of Ancient design, similar to the chair found on Praclarush Taonas. The chair is powered by a zero point module, or ZPM, a crystal power source that draws its energy from subspace. When O'Neill sat in the chair and summoned the power of the Ancients, thousands of glowing energy drones swarmed from the outpost into space and destroyed Anubis's fleet as it attacked Earth. The drones targeted and annihilated the Kull Warriors and all Goa'uld vessels in orbit above Earth, including death gliders, alkesh, motherships, and Anubis's own flagship, without harming the Prometheus or any Earth vessels.
The Ancient technology responds only to certain individuals who possess the Ancient gene and is controlled through the power of the mind, thus others could not draw upon its power as O'Neill had done, and following the defense of Earth, the power of the zero point module that had been brought back from Praclarush Taonas had been depleted. Although it is a formidable weapon which the Goa'uld fear, the technology has not yet been mastered, and Earth has not been able to use the weapon again to defend the planet.
In parallel universes, alternate SG teams have struggled to find an alternative power source for the Ancient chair to defend the planet against the Ori. In one instance, an alternate team attempted to enter our reality to acquire a ZPM from Atlantis. In another, the entire power grid of the US had been reconfigured to supply power to the chair, now housed at Area 51, but the power was used instead to take the planet out of phase. In our reality, the Ancient chair remains in Antarctica, and although the threat of an Ori attack has forced the IOA to consider bringing the Atlantis ZPM to Earth to power it, the Antarctic defense system is being held in reserve, and most enemies are unaware that Earth is currently without a viable power source to operate it.
Cross Reference: Ancients, Ancient Gene, Ancient Outpost, Ancient Repository, Ancient Stasis Chamber, Antarctica, Anubis, Praclarush Taonas, Zero Point Module
Episode Reference: Lost City