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"underground" is also used as a crossword clue. If that's what you saw, the answer is often subterranean (12 letters).
Definitions
beneath the surface of the earth
Word origin
From Middle English undergrounde (adverb), equivalent to under + ground or under- + ground. Compare Dutch ondergrond, ondergronds, German Untergrund, Danish undergrunds.
Used in a sentence
“There is an underground tunnel that takes you across the river.”
“One of the hidden glories of Victorian engineering is proper drains. Isolating a city’s effluent and shipping it away in underground sewers has probably saved more lives than any medical procedure except vaccination.”
“These criminals operate through an underground network.”
Source: Wiktionary, CC BY-SA 4.0
Used as a crossword answer1 curated clues
01“Below the surface”11 letters
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