The zero point module, or ZPM, is an Ancient crystal power source that draws its energy from subspace. The device was first discovered on Praclarush Taonas and brought back to Earth. At the Ancient outpost in Antarctica, SG-1 discovered a second ZPM which had long since become inert. By replacing the dead ZPM with the one retrieved from Praclarush Taonas, O'Neill was able to activate the Ancient defense weapon which annihilated Anubis and saved Earth.
The zero point module is the only thing known to be powerful enough to activate the Ancient defense weapon, however, following the battle with Anubis, its power was severely depleted, leaving Earth unable to use the weapon a second time. The device retained enough energy to activate the stargate and maintain a single connection to Atlantis in the Pegasus Galaxy, however after the Atlantis expedition departed, contact could not be reestablished. The SGC thought they had found a replacement for the burned out device when Camulus made the gesture of offering a ZPM in return for his freedom. However, gamma radiation caused the ZPM to glow, revealing an illuminated substance foreign to what is known to be the normal molecular composition of a ZPM. Camulus had deliberately tampered with the device, and the introduction of a constant electric charge would have caused an explosion capable of destroying not only the planet, but the entire solar system. O'Neill sent Earth's dead ZPM through the stargate with Camulus and retained the tainted device to prevent its use as a weapon against Earth, and the SGC continued to seek an active ZPM that could provide the necessary power to defend Earth and reach Atlantis.
When Catherine Langford died, she bequeathed her research to Daniel, and among her effects was an original 1889 edition of "The Eye of the Sun." The rare book contained a reproduction of a wall painting discovered by German archaeologists in 1885 which showed a figure holding what appeared to be a zero point module, a religious icon referred to by worshippers as "The Heart of Light." The painting suggested that during Ra's reign on Earth, he had possessed a ZPM without knowing its value as a power source, but extensive satellite sweeps of the Giza Plateau for the ZPM's characteristic energy signature indicated that the device was no longer on Earth. SG-1 made the decision to use the Ancient time travel device to journey back to ancient Egypt in the year 3000 BC to retrieve the ZPM, however they became trapped in the past and were unable to return. To ensure that the device would remain on Earth for future generations, SG-1 buried the ZPM in a tomb of the first dynasty near Giza which they knew would be uncovered shortly before their journey through time had begun. There the device remained for 5000 years until it was discovered by a team of archaeologists from the University of Chicago and returned to the SGC for study.
With history set right, the SGC now had a fully functional zero point module which could be used to power Earth's defenses or to open a wormhole to the Pegasus Galaxy. The device was used by Atlantis to maintain the city's cloak and to make it possible to open a wormhole back to Earth, but through contact with Atlantis, the SGC has had access to additional ZPMs. The Odyssey uses a ZPM power source which has not only given it intergalactic capabilities, but has also made cloaking technology possible.
Cross Reference: Ancients, Ancient Defense Weapon, Ancient Outpost, Ancient Time Travel Device, Atlantis, Camulus, Crystals, Egypt, Catherine Langford, Odyssey, Plant of P6J-908, Praclarush Taonas, Ra, Time Travel 3000 BC
Episode Reference: Lost City, Zero Hour, Moebius