Area 51

Area 51 is a top-secret U.S. military base.

Ultimate Alien
It was first mentioned in Framed and later appears in Prisoner Number 775 Is Missing. Lieutenant Steel works at (or at least knows about) Area 51.

Area 51 appears at the beginning of the episode Prisoner Number 775 Is Missing, where Old George arrives at this place, after knocking out the guards single-handedly, George entered one of the warehouses. A bright yellow light glows inside the warehouse that explodes causing Area 51 to completely disappear (as Cooper explained that there was no crystallized glass so it wasn't teleported and no scorch marks so it wasn't blown up). It is then discovered that there was a secret alien prison located underneath Area 51 that has been in commission by the United States Government for around 50 years, a facility designed to hold "alien combatants" on charges of being a possible threat to humanity which is a direct violation of both Plumber protocol and interstellar law.

Omniverse
A mysterious Galvan arrives and demands Blukic and Driba to give back a super powerful weapon needed to stop a threat to Earth.

Ben 10: Ultimate Alien

 * Prisoner Number 775 Is Missing (first appearance)
 * Solitary Alignment
 * The Beginning of the End

Ben 10: Omniverse

 * Blukic and Driba Go to Area 51 (first re-appearance)

Video Games

 * Ben 10: Protector of Earth

Notable Employees

 * Colonel Rozum
 * Lieutenant Steel
 * SECT
 * Max Tennyson (formerly)
 * Professor Paradox (formerly)

Notable Prisoners

 * Prisoner 775 (formerly)
 * 774 other aliens (formerly)
 * Blukic (formerly)
 * Driba (formerly)
 * Benevelon

Notable Visitors

 * Old George
 * Cooper Daniels
 * Ben Tennyson
 * Gwen Tennyson
 * Kevin Levin
 * Forever Knights
 * Luhley
 * B.L.R.R.T.