M5S-224 is a planet in the Pegasus galaxy. Its Stargate is located among a series of pillars in an area filled with perpetual fog, which contains enough energy to potentially connect the Stargate to the Milky Way galaxy if the control crystal from Atlantis is installed in its Dial Home Device.
Unknown to visitors, however, the "fog" actually consists of sentient lifeforms, and each activation of the Stargate destroys several of them with the unstable vortex and the amount of energy required to activate the Stargate, as they're what powers it.
In September 2004, an alien impersonating former SGC Commander, Major General George S. Hammond trapped the members of the First Atlantis Reconnaissance Team, Major John Sheppard, Dr. Meredith Rodney McKay, Teyla Emmagan and Aiden Ford as well as the Expedition's leader, Dr. Elizabeth Weir in a series of hallucinations. After learning the truth that they were not really back on Earth, the group discovered from the alien mist that if they attempted to use the Stargate to dial Earth, then many, many alien mists would be killed in the process. The group were eventually released after Weir convinced the mist Hammond that they would stay in Atlantis, Presumably, after returning to Atlantis, this address was permanently removed from Atlantis's database. (SGA: "Home")
- In an alternate reality, the energy beings of M5S-224 likewise deceived the First Atlantis Reconnaissance Team and Dr. Weir into believing that they had successfully returned to Earth. After arriving in our reality, the Lt. Colonel Cameron Mitchell of this reality cited this incident as evidence in supporting of his theory that the Stargate Command of our reality was an elaborate alien deception. However, it was later revealed that the SG-1 of this reality were responsible for the convergence of the various realities. (SG1: "Ripple Effect")