Atlantis' shield

"The people who built this city knew they had a forcefield capable of holding back anything Mother Nature, or for that matter the Wraith, could throw at them. Without that protection, Atlantis is remarkably fragile."

- Rodney McKay

The shield of Atlantis serves as the main (and only) defense for the Ancient city-ship, and is largely considered to be the most powerful shield ever created.

Functions
Activated by ten emitters located throughout the city, the shield is used to protect the city from everything trying to harm it, including weapons-fire, ocean pressure and even solar and nuclear radiation and asteroids. It also is a critical part when the city takes off, lands or flies through space, since it's the shield which prevents the air from leaving the city, thus acting as a sort of outer hull.

The shield requires massive amounts of energy to be activated, which usually can only be provided by a Zero Point Module; however, it has been shown that the energy of several lightning strikes provides a sufficient amount of power to activate the shield for some time. ZPM's are sufficient to power the shields for at least several hundred years.

The shield of Atlantis is unique in the sense that no residual energy can get past the shield (Atlantis cannot suffer so much as a scratch whilst the shield is activated unlike the shields of starships where systems are damaged and damage is sustained regardless of the shield's status). The effectiveness of the shield does not deplete under external forces, and will literally remain impenetrable as long as sufficient power exists to hold back the forces putting pressure on it. This means nothing can penetrate the shield as long as there is a constant power source (again unlike starships where power can still be active with the shields loosing integrity and needing to recharge). If there is no pressure on the shield it can remain active for an indefinite amount of time.

Usually the shield is used to protect only the city and a part of the surrounding area, however it can be expanded to cover a significant portion of an average-sized planet or even shrunken to encompass individual objects inside the city; it has been proven that altering the size of the shield burns far more energy that what would typically be used up, and also reduces the shield's effectiveness. This ability was used once by the Atlantis Expedition to surround the Stargate when it exploded; however the strength of the explosion was greatly amplified when the shield compressed the explosion and the massive amount of energy (nearly 12 naquadah-enhanced nuclear explosions) inside the shield caused the emitters to fry out and the shield to collapse, releasing the remaining energy.

Other uses
Over time the Atlantis Expedition has discovered alternative uses for the shield of Atlantis. The such time was during the Wraith attack on Atlantis. To trick the Wraith into believing Atlantis was destroyed, the Expedition converted the shield into a cloak, turning the whole city invisible to both visual and mechanical means of observation. The downside to using the shield in this way is that the cloak replaces the shield entirely, meaning the city is vulnerable to an attack if an enemy isn't fooled.

After the Asurans took over Atlantis Rodney McKay developed a means to convert the entire shield into a massive anti-Replicator field: by linking the primary control crystal of an Anti-Replicator gun into all ten shield emitters, every Replicator in the city would be destroyed when the shield was activated.

Weaknesses
The shield of Atlantis is designed to allow any Ancient vessel, such as gateships, to pass through it, which they accomplish by generating an unknown form of electromagnetic field. Unfortunately, any race with access to an Ancient ship and a reasonable understanding its technologies, such as the Lost Tribe, would easily be able to duplicate the effect and fly a ship straight through the shield. After a mysterious ship stole part of the Attero device and kidnapped Drs. McKay and Jackson, the shield was reconfigured to prevent this from ever happening again. It is unknown if gateships will be able to pass through the shield in the future if needed, or if the shield must be temporarily reset whenever such a situation arises. A much less obvious weakness of the shield is that it does not encompass the city simulaneously when activated, allowing attacks to pass through the hole in the shield before it completely closes. This weakness was discovered when the satellite launched by the Asurans managed to damage the city while it was escaping from the planet.