Atlantis' shield

"Fortunately there is some sort of a forcefield holding back the w...water."

- Rodney McKay

The shield of Atlantis is a dome-shaped shield that 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
"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

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 of the city whenever its stardrive is used, 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 is usually provided by a Zero Point Module; however, it has been shown that the energy of several lightning strikes will provide an amount of energy sufficient to power the shield for some time. A single ZPM is sufficient enough to power the shield for at least 3.3 thousand years, whilst holding back several thousand feet of water.

The shield of Atlantis is unique in the sense that its effectiveness does not deplete under external forces, and will literally remain impenetrable so long as sufficient power exists to hold back the forces putting pressure on it. In other words, nothing can penetrate the shield as long as there is enough power to sustain it (unlike starships, which deplete under external forces and slowly loose integrity no matter how much power is available). If there is no pressure on the shield it can remain active for an indefinite amount of time.

While the shield is typically used to protect only the city and the immediately surrounding area, it can be expanded to cover a significant portion of an average-sized planet or even shrunken to encompass individual objects inside the city itself. Using the shield in this way, however, burns far more energy that what would typically be used up, and also reduces the shield's effectiveness. This ability was used by the Atlantis Expedition to surround the Stargate when it exploded due to the effects of the Attero device; 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 and destoying a large portion of the surrounding area.

Other uses
"By interfacing a Jumper's stealth cloak generator with the city's shield, we should be able to render the city invisible."

- Rodney Mckay

Over time the Atlantis Expedition has discovered alternative uses for the shield of Atlantis. The first such time was during the Wraith attack on Atlantis. In order to trick the Wraith into believing Atlantis was destroyed, the Expedition converted the shield into a massive cloak, turning the whole city invisible to both visual and mechanical means of observation. When the shield used in this manner the emitters draw much less power than normal, allowing the cloak to remain active for far longer than the shield could when under no stress. The downside to using the shield in this way was that the cloak is simply the shield operating on another frequency, meaning the shield is replaced entirely and the city is vulnerable to attack if an enemy isn't fooled, as it takes time to reconfigure the cloak back to a shield. The Atlantis Expedition has since been able to reduce this time to only a few seconds.

"...if we can interface the crystals from the disrupter weapons with those of the shield emitters, we should be able to trigger a massive anti-Replicator wave throughout the city."

- Rodney Mckay

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

Weaknesses
Despite being impenetrable to any means of attack while power is available, the shield of Atlantis is designed to allow any Ancient vessel, particularly gateships, to pass through it by generating an unknown form of electromagnetic field. Unfortunately, any sufficiently advanced race with access to an Ancient ship equipped with this mechanism and a reasonable understanding of Ancient technology would easily be able to duplicate this bypass effect and fly an object straight through the shield. After a mysterious ship stole part of the Attero device and kidnapped both Dr. McKay and Dr. 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.

Another weakness of the shield is that it does not encompass the city simulaneously when activated, instead from the bottom up and from the edges of the city inward, allowing a brief window for attacks to damage the city by passing 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, causing it to drop out of hyperspace prematurely.

A much less obvious weakness is that, if the city is using its stardrive, particiularly intense weapons fire will exert enough force to actually alter the city's trajectory to an extent that the inertial damperers cannot properly compensate. When this happens damage to systems that are not secured as well as on normal starships is sustained, as the city was not meant for space combat.