The SGC is a highly secure facility, and various levels of coded emergencies are used in the event of invasion or containment failure.
When the nanites of Orban were discovered on base, the facility was put on a Threat Con Bravo alert.
A Class One security lockdown was issued when the inter-dimensional beings became visible on the base, requiring that the facility be shut down and a security sweep completed before returning to normal operating status.
When Daniel returned from the Stromos carrying the personalities of a dozen individuals within his mind, Dr. Fraiser declared a Code 17 emergency, requiring him to be kept under constant guard with limited direct contact with base personnel.
A Code 9 lockdown, requiring a quarantine of the base, was declared when the SGC was infiltrated by the Spirits, by the Reetou, by the Entity, by the Replicators, and when O'Neill was suddenly and unexpectedly transported aboard Thor's ship.
A Code 5 lockdown, initiated during the threat of the histaminolytic virus, mandates that the mountain be sealed and anyone attempting to leave be shot on sight.
In the event of containment failure, such as the infiltration by the organisms of the orb, the Wildfire Directive activates automatically, requiring level four quarantine, internal power and life support, nothing entering or leaving the mountain, and initiation of the autodestruct sequence.
During the foothold scenario drill for SGC trainees, a number of emergency situations were declared, including Code Foxtrot Alpha Six. The training scenario programmed into the virtual reality chair during Teal'c's test of the system also used the Foxtrot Alpha Six threat level.
When the Ba'al clones escaped from the interrogation room on level 16, Landry ordered a full security lockdown on levels 15 through 17. When the non-corporeal entity of Anubis gained access to the base and possessed SGC personnel, O'Neill ordered that the base be put on lockdown and the mountain sealed, and all off-world teams were directed to the Alpha Site. The lockdown lasted for six days before the President ordered that normal gate operations must be reestablished, and O'Neill devised a ruse to force Anubis to reveal himself.
When Replicators invaded the SGC through the stargate, O'Neill ordered a base lockdown including the emergency evacuation of all personnel through alternate routes and the initiation of the autodestruct protocol. The base was sealed, however the autodestruct could not be initiated. Instead the remaining personnel attempted to evacuate through the stargate, an emergency state of alert was issued for the evacuation of a 200 square mile radius around Cheyenne Mountain, and the President authorized the deployment of a 10-kiloton nuclear weapon to be dropped down the silo and detonated on O'Neill's command. The Replicators were defeated before the destruction of the facility was necessary.
Cross Reference: Authorization Codes, Autodestruct Sequence
Episode Reference: The Broca Divide, Message in a Bottle, Spirits, Show and Tell, Fair Game, Learning Curve, Entity, Proving Ground, Menace, Sight Unseen, Lifeboat, Lockdown, Avatar, Gemini, Reckoning, Insiders