| enigmatical(obscure, deep, unknown, clandestine, underground) |
| esoteric(adjective, obscure, unknown, clandestine, underground) |
| furtive(adjective, obscure, unknown, clandestine, underground) |
| hush-hush(adjective, clandestine, underground, undisclosed) |
| mystic(adjective, obscure, unknown, clandestine, underground) |
| obscure(adjective, unknown, clandestine, underground, undisclosed) |
| out-of-the-way(obscure, deep, unknown, clandestine, underground) |
| recondite(adjective, obscure, unknown, clandestine, underground) |
| reticent(obscure, deep, unknown, clandestine, underground) |
| retired(obscure, deep, unknown, clandestine, underground) |
| secretive(adjective, clandestine, underground, undisclosed) |
| shrouded(adjective, obscure, unknown, clandestine, underground) |
| silent |
| sly(adjective, clandestine, underground, undisclosed) |
| stealthy(adjective, clandestine, underground, undisclosed) |
| surreptitious(adjective, clandestine, underground, undisclosed) |
| tacit |
| unbreathed(adjective, latency implication) |
| undeclared |
| under cover(expression, adjective, deep) |