Area 51 is a top-secret U.S. military base.
Ben 10: Ultimate Alien
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 intersteller law.