| stealthy(clandestine, underground, undisclosed) |
| strange(clandestine, underground, undisclosed) |
| sub-rosa(secret, clandestine, underground, undisclosed) |
| supersecret |
| surreptitious(secret, clandestine, underground, undisclosed) |
| top secret(expression, secret, clandestine, underground, undisclosed) |
| unacknowledged(clandestine, underground, undisclosed) |
| under-the-table(secret, clandestine, underground, undisclosed) |
| under false pretense(expression, idiom, clandestine, underground, undisclosed) |
| under security restrictions(expression, idiom, ranked) |
| undercover(secret, clandestine, underground, undisclosed) |
| underground(clandestine, undisclosed) |
| underhand(clandestine, underground, undisclosed) |
| underhanded(clandestine, underground, undisclosed) |
| unenlightened(clandestine, underground, undisclosed) |
| unfrequented(clandestine, underground, undisclosed) |
| unintelligible(clandestine, underground, undisclosed) |
| unknown(clandestine, underground, undisclosed) |
| unpublished(clandestine, underground, undisclosed) |
| unseen(clandestine, underground, undisclosed) |