From The Stargate Omnipedia
A powerful technology created by the Ancients
and incorporated into the Atlantus outpost
, deep beneath Antarctica's frozen surface (and likely on Proclarush
, as well).
A user sits in the outpost's command chair to control the weapon. The device appears to interface directly with the user's mind, allowing one to select a target to be destroyed. Countless quantities of Ancient drones emerge from under the outpost and stream toward the target, not unlike a swarm of bees, cutting through even the most powerful Goa'uld shield technology.
The super-weapon formed the basis of Earth's defense platform, which provided a valuable trump card against enemy attack for many years. Due to international non-proliferation treaties covering the Antarctic continent, the chair and weapon were eventually relocated to Area 51 in the United States. Shortly thereafter it was destroyed by a Wraith Dart kamikazee run during an attack on Earth.
Lost City, Part 2 - O'Neill interfaces with an Ancient console deep beneath the surface of Antarctica, activating the Ancient super-weapon they left behind. Anubis's invasion fleet is destroyed, saving Earth.
New Order, Part 1 - The S.G.C. hosts three Goa'uld System Lords, who recognize that the newly-discovered super-weapon prevents them from attacking Earth.
Enemy At the Gate - A Wraith hive ship attacks Earth, successfully destroying the weapons platform with a wave of Darts and intel on its location received from a parallel reality.