Underground

Underground is a term used to denote a secret or hidden organisation, usually of a subversive nature, although the subversive character is often very point-of-view dependent. The term is obviously derived from the parallel with mining, and undermining of defences, both activities which proceed with minimal disturbance to the surface.

Some famous underground organisations were:
 * The Underground Railroad ferried slaves to the nonslave states of the U.S. and Canada before slavery was outlawed.
 * Homegrown resistance to occupying armies is often referred to as "the Underground," particularly in reference to the Western European countries in World War II under the Axis, where the undergrounds with the covert support of the Allies initiated sabotage and recovery of Allied war prisoners and shotdown aircrew.
 * The Weather Underground was a U.S. domestic terrorist group active in the 1960's and early 70's, who sought to overthrow the U.S. government.
 * The London Underground is an underground train system in London, England, although it is seldom not at all affiliated with covert, revolutionary or otherwise illegal operations.
 * The "underground economy": pretty much anything that is not taxed or regulated by the government, whether legal (yard sales and street vendors), of questionable legality (grey market importers, unlicensed contractors, undocumented immigrant labor), or illegal (drugs, prostitution).

The term is not now commonly applied to specific groups, but is more often applied to suspected groups as a scare tactic, as labelling the boogeyman an underground organisation means you don't actually have to present any evidence that they actually exist -- in a reversal of logic, absence of evidence is considered evidence of how well organised the underground movement is. That is not to say that underground movements do not exist, just that the term underground is not normally used if they are known about -- which stands to reason if you think about it.