The Danger Room is a sub-level room where the students at the X-Mansion can train and practice/hone their powers through holographic simulations.
Due to the unstable and dangerous nature of his powers, Alex Summers was trained by Charles Xavier in a nuclear bunker located beneath the Xavier Mansion. The bunker was built by Charles' stepfather for his family's safety due to the advent of the nuclear age and the likelihood of a nuclear war breaking out.
Wolverine used the Danger Room in order to simulate a fight with Sabretooth on Liberty Island. Iceman would later use it to practice his powers by simulating a fight with the Sentinels at the aforementioned location.
- In an alternate script for X-3 written by Michael Dougherty the Danger Room was built by Cyclops due to his guilt over Jean Grey's death. He felt the X-Men were too weak and too untrained, which resulted in her death so he built the room to make them a stronger team. 
- In the original script for X-Men 2:United, Logan was going to perform drills in the Danger Room when he couldn't sleep. This is show cased in the novel of the film, and the comic book adaptation of the film's original script, as Logan is shown working out his rage. David Hayter stated after being let go from Fox that the only reason the Danger Room was cut from the film was due to it being estimated the scene in question would've gone over-budget.