| ticklish(hard, delicate, complex, tricky, complicated) |
| transcendental(strange, supernatural, spiritual, mystical, inexplicable) |
| unaccountable(solid, supernatural, dense, spiritual, mystical) |
| uncanny |
| uncertain(vague, unclear) |
| unclear(hard, obscure, delicate, uncertain, vague) |
| uncomprehensible |
| unenlightened(obscure, deep, unknown, clandestine, underground) |
| unexplained |
| unfrequented(obscure, deep, unknown, clandestine, underground) |
| unknowable(solid, strange, supernatural, dense, spiritual) |
| unknown(obscure, strange, clandestine, underground, undisclosed) |
| unnatural |
| unpublished(obscure, deep, unknown, clandestine, underground) |
| unsearchable |
| unseen(obscure, unknown, clandestine, underground, undisclosed) |
| vague(obscure, uncertain, unclear, indefinite, equivocal) |
| veiled(obscure, strange, unknown, clandestine, underground) |
| vexing(hard, delicate, complex, tricky, complicated) |
| visionary(supernatural, spiritual, mystical) |