The Vulcan Underground was a group established in the mid to late 23rd century in the mirror universe. It was formed by Yevok when he came to the conclusion that no progress could be achieved as long as T'Pau remained in power. Thus, he worked alongside Spock and for four years worked to cultivate the Underground. This consisted of Vulcans serving T'Pau adnd gaining the commitment of hundreds of their ranks with the intention of switching sides at the appointed time.