The Plastic Prison was a prison designed to incarcerate Magneto.
Biograghy
X-Men
Following the battle on Liberty Island, Magneto was arrested and sent to the Plastic Prison, designed by William Stryker (possibly through the Weapon X program).
X2: X-Men United
Stryker made several visits to Magneto's Plastic Prison, brainwashing and interrogating him in order to extract information regarding Cerebro.
Mystique seduced Mitchell Laurio, Magneto's prison guard, and injected him with iron particles. Magneto later extracted the iron out of the Laurio's body, which killed him. He then destroyed the Plastic Prison, using spherical projectiles he formed from the iron, and escaped.