The word underground is a noun. Underground means (1) electric underground railway, (2) a secret group organized to overthrow a government or occupation force.
Underground is also an adjective that means (1) used of independent armed resistance forces, (2) under the level of the ground.
Underground is also an adverb that means (1) beneath the surface of the earth, (2) in or into hiding or secret operation.
The following articles and web pages elaborate on the word underground.
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