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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Geophysicists reported that Earth's ever-shifting, underground network of tectonic plates was firmly in place more than 4 billion years ago -- at least a billion years earlier than scientists generally thought.
Science Daily. Wednesday, 27 May 2020 12:33:34 EDT.
The White House is considering restarting underground nuclear tests, ending a moratorium that began after the Cold War ended. It's not only dangerous and unnecessary. It's nuts.
Doyle Mcmanus. Los Angeles Times. Wednesday, 27 May 2020 07:00:47 -0400.