The aliens of P3X-118 established a foothold situation, or alien incursion, on Earth when they arrived through the stargate in the guise of SG-6 and secretly took command of the SGC. Preparing to invade Earth as a viable new homeworld, they hid their activities by closing off Level 23 because of a supposed tetrachloroethylene rupture, a gas which causes hallucinations and paranoid delusions.
Small electronic devices worn by the aliens could duplicate the appearance and access the thoughts of SGC personnel, who were kept alive in suspended animation on Level 23. Duplicates could access the knowledge, personality, and behavior patterns of their human counterparts, who remained unconscious until an alien was killed and the connection was severed. Among those who were duplicated were O'Neill, Daniel, Hammond, Fraiser, and Major Davis, however the procedure failed to work on Teal'c and Carter because their body chemistry had been altered by the presence of a symbiote.
A sustained specific tone would disrupt the devices, making the alien form visible. The alien incursion was averted, and as they were defeated, many aliens escaped through the gate, possibly retaining the knowledge of their human counterparts. Because of the seriousness of the security breach, a presidential order erased all mention of the foothold incident from official records. To help guard against future such incidents, a foothold situation has also been used as part of the SGC training scenario for graduates of the Air Force Academy.
Cross Reference: Paul Davis, Hazmat, Harry Maybourne, Mimic Device, P3X-118
Episode Reference: Foothold, Proving Ground