An underground was a group of dissidents or resistance fighters who opposed to the ruling party of any level of a group or government's control.
An underground movement, which operated in groups of three, existed on Beta III. This group consisted of natives who were not affected by the control of the Body created by Landru. (TOS: "The Return of the Archons")
An underground opposed to the Nazi party on Ekos was in full effect in 2268.
This group consisted of both Ekosians, who were opposed to the changes to the government brought about by Deputy Führer Melakon, and Zeons from the sister planet Zeon. As members of this movement did not know who their leaders were, if they were captured by the Gestapo, the damage would be limited to only a handful of indivdiuals harmed with the larger group going unscathed.. They had successfully infilitrated the Ekosian military and counted among its numbers several top officials, including Daras. After Melakon was assassinated, this group led the effort at bringing peace to both worlds. (TOS: "Patterns of Force")