Fort Knox is an army base, located in Kentucky. It is well known for being the location of the United States Bullion Depository. It is also an old Plumber Base which was decommissioned from service. It was used as a battleground during one of Max Tennyson's confrontations with Vilgax in the past.
In Ben 10 vs. the Negative 10: Part 1, Fort Knox was one of the two bases that contained the keys required to open the force field at Mt. Rushmore. The base was attacked by members of the Negative 10 and the base's key was stolen.