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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They show the start of construction at the Natanz nuclear facility of what may be an underground replacement for a sabotaged advanced centrifuge assembly plant.
CBS News. Wednesday, 28 Oct 2020 06:58:14 -0400.
The U.N.'s International Atomic Energy Agency confirmed that Iran is rebuilding an underground advanced centrifuge assembly plant, and that the agency had prior knowledge of the construction.
CNBC. Wednesday, 28 Oct 2020 10:54:41 GMT.