Base left behind under the ice of Antarctica on Earth after the Ancients departed for the Pegasus Galaxy with the city of Atlantis. One Ancient, later named Ayiana by her modern-day rescuers, was left behind to fend for herself in this outpost.
The outpost contained at least one depleted Zero Point Module, originally capable of calling upon thousands of Ancient drones that make up the Ancient super-weapon, capable of protecting the planet from invasion or other threats. It was found near where Earth's original Stargate was located, and where Ayiana was found frozen in the ice.
After the outpost was discovered and its powerful drone weapons were used to destroy Anubis's invasion fleet, an international research team set up an on-going study of the facility and its technology. The team was headed by Elizabeth Weir, and included such scientists as Rodney McKay, Peter Grodin, and Carson Beckett. After Daniel Jackson discovered the gate address to Atlantis' new home while at the outpost, Weir led her expedition team through the gate. Presumably, scientists from many countries continue to study Atlantus.
Lost City, Part 2 - O'Neill configures a Tel'tak to burrow down to the Ancient outpost to use its weaponry to save Earth from Anubis.
Rising, Part 1 - Dr. Weir, who has since been assigned to the outpost to unearth information regarding the Ancients, learns from Dr. Jackson that the coordinates to reach Atlantis have been found.
Ripple Effect - An alternate Cameron Mitchell confesses that his people's Z.P.M. is depleted. They need a charged module to power Atlantus outpost and defend Earth from a new threat.