The main training center for the X-Men, located three levels down in their basement. The original Danger Room had simple and complex mechanical obstacles and tests that were used by the team to hone their powers. After meeting the Shi'ar and helping Lilandra to regain her throne (the first time), the Danger Room was refitted with advanced Shi'ar technology, which allows it to create almost any scenario and setting imaginable. It can even create simulacra of people, as well as smells and sounds. A combination of advanced animatronics and forcefields gives the holograms a realistic feel, and allows the X-Men to prepare for almost any eventuality.
The Danger Room's control center opened out onto the main rectangular area itself, and allowed controllers to monitor the exercises and abort them if necessary. The holograms and robots were equipped with failsafes and safety protocols to prevent training accidents, but if these failed the room would become incredibly dangerous. During Operation: Zero Tolerance the Danger Room was picked clean by nannites, so it was not in use for a while. However, when Professor X returned the Danger Room was refurbished and made operational again.
After Magneto IV destroyed the school buildings, the X-Men rebuilt the Danger Room in a circular configuration, with a control center in the ceiling instead of on the wall. This Danger Room has all the holographic capabilities of its predecessors, and probably more. However, an encounter with the depressed Wing, who had been "cured" of his mutation by the alien Ord, allowed the Danger Room to cirvumvent its failsafes and kill. After imprisoning the students, it tricked the X-Men into destroying the computer core housing, freeing it to become Danger, a robotic AI entity who nearly killed the X-Men. Danger had the ability to communicate with and control any technology, and even bring advanced technology to life. Upon her defeat, she revealed to the X-Men that Professor X had known she was sentient for years, but ignored her telepathic calls and kept her prisoner inside the Danger Room's machinery. A new Danger Room has yet to be built, but the room is still used as a training area.