Atlantis' shield

The shield of Atlantis is the main defense for the Ancient City-ship.

Functions
The shield is used to protect the city from everything trying to harm it, including weapons-fire, ocean pressure and even solar 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 needs massive amounts of energy to be activated, which is usually only provided by a Zero Point Module. It has however been shown that the energy of several lightnings provides a sufficent amount of power to activate the shield for some time. If there is no pressure on the shield it can be activated for a nearly unlimited amount of time.

The shield is activated by ten shield emitters, located throughout the city.

Alternative uses
Over time the Atlantis Expedition has found out different ways of using the shield of Atlantis. The first different use was during the Wraith attack of Atlantis. To trick the Wraith into believing Atlantis was destroyed they were able to convert the shield into a cloak which hides the whole city, in the same manner as the cloak of a Puddle jumper can be converted into a shield. However if this is done, the cloak replaces the shield, making the city vulnerable for attacks if an enemy isn't tricked by this.

While the Asurans took over Atlantis Rodney McKay came up with the idea to convert the whole shield into a gigantic Anti-Replicator Gun. This was done by linking up the energy crystals of the ARGs to the shield emitters. When the shield was the activated, all Replicators in the city were destroyed. For this plan to work, all shield emitters had to be changed in this way.