Cryo Prison
The Cryo Prison was a location in Millennium City that was used by Time Force to store captured Mutants. There's also a special containment unitwhere the prison keeps its most dangerous mutants called the X-Vault.
In the year 3000, Ransik used the prison to travel into the past and, once in the 21st Century, used it as a base and gradually released all of the captured Mutants. The Cryo Prison returned to the future between "The End of Time: Pt. 3" and "Reinforcements from the Future: Pt. 1" as it appeared in the latter.