In Alan Moore's 1963 miniseries, Mount Rushmore is used as the Tomorrow Complex, the secret headquarters for the superhero team the Tomorrow Syndicate.
The Rushmore Sanctuary is Mr. Majestic's secret base. It's outfitted with lots of Kherubim technology that he landed with as well as things he's created since coming to Earth. Some of Majestic's more impressive technology are his computers. Majestic's main machine is a Kheran difference engine, a computer that supposedly taps every computer on every planet including scores of alternate Earth's and is powered by a battery two galaxies away the size of a small moon. It's programmed to constantly upgrade its intelligence and its processing speed is so fast it can go through lifetimes worth of information in an hour. The Sanctuary also has a doorway to Otherspace, a transubstantiation tube where Majestic can transmute objects and living beings, a forge for Majestic to create new technology, such as Planet Movers, and an armory for Majestic's weaponry. To protect all this and more, the Sanctuary is completely undetectable, cannot be entered by teleportation, and is outfitted with Kheran kill-ware and war-ware.