Some Ancient technology requires Z.P.M.s, and the city of Atlantis relies on three of the units for its primary power generation. A Zero Point Module or equivalent power source is required for a Stargate to establish a wormhole to a gate in another galaxy. It is possible to construct a version of the device from Earth-based materials, as Jack O'Neill (under the influence of the Ancients' library of knowledge) once did to reach the Asgard when he needed their aid.
SG-1 first discovered the technology when O'Neill, again under the influence of the Ancients' knowledge, led them to the planet Proclarush to retrieve a Z.P.M. from Taonas, the Ancients' abandoned base there. They brought it back to Earth and found the Atlantus outpost in Antarctica, where the device powered the Ancient drone weapons that destroyed Anubis's attacking fleet. The module was later used to power the Stargate for the Atlantis expedition's long-distance journey to the Pegasus Galaxy.
Upon the Earth team's arrival in Atlantis two of the city's three Z.P.M.s had been depleted, and the third was reaching maximum entropy trying to stave off the ocean around the submerged city with a shield barrier. It is the Atlantis team's hope that additional Z.P.M.s can be discovered in the Pegasus Galaxy, both to power the city's defense shield and to provide enough power to gate home to Earth.