| resistor | |
| sly(clandestine, undisclosed) | |
| sneaking | |
| sotto voce | |
| sub rosa(expression, idiom) | |
| submarine | |
| submerged | |
| subterrestrial(adjective, surface) | |
| subversives | |
| train | |
| tubing | |
| tunnel | |
| unconventional(adjective, secret, subversive) | |
| under-the-counter(hidden, fraudulent) | |
| under-the-table(secret, clandestine, undisclosed) | |
| under cover(expression, secret, spy, covert) | |
| underneath(secret, hidden, spy, covert) | |
| underpass | |
| undisclosed(adjective, obscure, secret, unknown, hidden) | |
| unknown(obscure, secret, hidden, concealed, unseen) |
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| APA | Smart Define. (2015). underground thesaurus. Retrieved September 21, 2026, from https://smartdefine.org/underground/thesaurus/synonyms |
| Chicago | Smart Define. 2015. "underground thesaurus" https://smartdefine.org/underground/thesaurus/synonyms (accessed September 21, 2026). |
| Harvard | Smart Define 2015, underground thesaurus, Smart Define, viewed 21 September, 2026, <https://smartdefine.org/underground/thesaurus/synonyms>. |
| MLA | Smart Define. "underground thesaurus" 3 November 2015. Web. 21 September 2026. <https://smartdefine.org/underground/thesaurus/synonyms> |