The autodestruct sequence is used as a failsafe measure to destroy the SGC complex in the event of invasion or containment failure. The SGC has needed to activate this sequence during a number of emergencies including infiltration by the organisms of the orb, and by the Replicators, although in each case the crisis has been averted before the countdown was completed. The sequence can be set locally using a key access, or by entering the proper code into the computer. Special authorization codes are required from two ranking officers to override the sequence. Similar security measures are in place at off-world sites as well. The Alpha Site was destroyed by a single radius blast when the autodestruct sequence was initiated during an invasion by Anubis's Kull Warriors. The Gamma Site was also completely destroyed when the autodestruct was triggered after containment failure involving the R75 insects which overran the base and escaped from the facility.
Cross Reference: Authorization Codes, Security Lockdown
Episode Reference: The Enemy Within, There But For the Grace of God, Prisoners, Message in a Bottle, A Matter of Time, Menace, Death Knell, Lockdown, Avatar, Reckoning, Threads, The Scourge