In Secrets, Mt. Rushmore was revealed to be a Plumbers base by Max. It was shown to have multiple pieces of technology, including an arsenal of heavy weapons, all of which could be accessed by pulling open different drawers. This technology was used to enter Vilgax's ship and rescue Ben. In Truth, Mt. Rushmore was visited by Phil so he could steal the Null Void Projector. This was because he was making money off of his so-called "heroics," (saving people from aliens), but he was actually releasing them so he could reimprison them to earn money. He, however, was stopped by Ben, Gwen and Max and sent into the Null Void.
In Ben 10 vs. the Negative 10: Part 1, and in Ben 10 vs. the Negative 10: Part 2, Mt. Rushmore was seen, however only the outer location was seen at the end of Part 1. In Part 2, the Tennysons tried but failed to prevent the Negative 10 from breaching the stronghold. They battled inside the room with the Sub Energy but though they defeated and trapped 8 of the Negative 10 members, they failed to protect the sub energy. However, Perk Upchuck stopped the Forever King by swallowing the energy and making a powerful blast. However, this destroyed Mt. Rushmore and a hologram was used to prevent anyone from seeing the damage.
Five years prior to Omniverse
Mt. Rushmore became a fully functioning Plumber base.
In Evil's Encore, Dr. Animo took control of the base and tried to use the Plumber satellite network to spread his mutation ray over the Earth. He was stopped by 11-year-old Ben, Gwen and Max Tennyson.
In The Ultimate Enemy: Part 2, Mt. Rushmore was damaged again by Vilgax (by ripping off George Washington's face to break into the base) and the Esoterica. It is unknown if it was repaired after this.