| close(obscure, secret, unknown, hidden, concealed) | |
| closet(obscure, unknown, hidden, clandestine, fraudulent) | |
| depth(noun, deepness) | |
| disorderly | |
| extremists | |
| guerilla | |
| immunity | |
| impedance | |
| in secret(expression, idiom) | |
| insurgents | |
| insurrectionists | |
| invisible | |
| mystic(adjective, obscure, secret, unknown, hidden) | |
| mystical(adjective, obscure, secret, unknown, hidden) | |
| nonconformist | |
| obscure(secret, unknown, hidden, concealed, unseen) | |
| obscured(adjective, secret, hidden, clandestine, concealed) | |
| railroad | |
| railway | |
| recondite(obscure, secret, unknown, hidden, concealed) |
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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> |