70 entries under underground topic. ×

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top secret(expression, clandestine, undisclosed)
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unacknowledged(clandestine, undisclosed)
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under-the-table(clandestine, undisclosed)
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under false pretense(expression, idiom, clandestine, undisclosed)
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under wraps(expression, clandestine, undisclosed)
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undercover(clandestine, undisclosed)
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underhanded(clandestine, undisclosed)
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unintelligible(obscure, deep, unknown, clandestine)
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unseen(obscure, unknown, clandestine, undisclosed)
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veiled(obscure, unknown, clandestine, undisclosed)
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APASmart Define. (2015). unpublished thesaurus. Retrieved September 26, 2026, from https://smartdefine.org/unpublished/thesaurus/synonyms
ChicagoSmart Define. 2015. "unpublished thesaurus" https://smartdefine.org/unpublished/thesaurus/synonyms (accessed September 26, 2026).
HarvardSmart Define 2015, unpublished thesaurus, Smart Define, viewed 26 September, 2026, <https://smartdefine.org/unpublished/thesaurus/synonyms>.
MLASmart Define. "unpublished thesaurus" 14 April 2015. Web. 26 September 2026. <https://smartdefine.org/unpublished/thesaurus/synonyms>