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.
Members of this faction pretended to serve T'Pau but in actuality worked to weaken her powerbase. This was with the intention of installing Spock as the ruler of Vulcan and the Terran Empire. However, the Underground greatly feared exposure from not only T'Pau but Spock's other enemies inside the Empire. (Decipher module: Through a Glass, Darkly)