70 entries under underground topic. ×

10
private(adjective, separate, simple, special, individual)
9
classified(adjective, close, show, secret, hidden)
9
secret(adjective, close, secret, private, hidden)
6
hush-hush(adjective, close, secret, hidden, clandestine)
5
confidential(adjective, close, show, secret, private)
4
arcane(adjective, secret, clandestine, undisclosed)
4
close(adjective, light, slight, fine, poor)
4
hidden(clandestine, undisclosed)
4
secluded(adjective, separate, simple, special, individual)
3
clandestine(adjective, secret, undisclosed, top-secret)
3
covert(adjective, secret, clandestine, undisclosed)
3
inside(adjective, secret, private, clandestine)
2
cryptic(clandestine, undisclosed)
2
esoteric(secret, clandestine, undisclosed, confidential)
2
furtive(adjective, secret, clandestine, undisclosed)
2
mysterious(clandestine, undisclosed)
2
occult(clandestine, undisclosed)
2
stealthy(adjective, secret, clandestine, undisclosed)
2
strange(adjective, separate, simple, special, individual)
2
top-secret(adjective, secret, clandestine, undisclosed)
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APASmart Define. (2016). restricted thesaurus. Retrieved September 22, 2026, from https://smartdefine.org/restricted/thesaurus/synonyms
ChicagoSmart Define. 2016. "restricted thesaurus" https://smartdefine.org/restricted/thesaurus/synonyms (accessed September 22, 2026).
HarvardSmart Define 2016, restricted thesaurus, Smart Define, viewed 22 September, 2026, <https://smartdefine.org/restricted/thesaurus/synonyms>.
MLASmart Define. "restricted thesaurus" 26 February 2016. Web. 22 September 2026. <https://smartdefine.org/restricted/thesaurus/synonyms>