| unbeatable(adjective, invulnerable) |
| unknowable(adjective, obscure, solid, dark, superior) |
| abstruse(adjective, obscure, difficult, strange, deep) |
| cloudy(adjective, dark, gloomy, dense, misty) |
| compressed(adjective, set, hard, tough, firm) |
| hazy(adjective, opaque) |
| perplexing |
| puzzling(adjective, obscure, strange, sophisticated, complicated) |
| rocky(adjective, set, firm, stiff, compact) |
| steely(adjective, firm, rigid, unyielding) |
| unaccountable(adjective, obscure, solid, strange, doubtful) |
| adamant(adjective, firm, rigid, unyielding, steely) |
| baffling(adjective, obscure, solid, deep, mysterious) |
| cryptic(adjective, obscure, strange, sophisticated, complicated) |
| dark(adjective, feature, obscure, mysterious, hidden) |
| hidden(adjective, obscure, difficult, solid, dark) |
| immune(adjective, unaffected, unyielding, unbending, resistant) |
| incognizable(adjective, incomprehensible, unintelligibility) |
| nonporous(hard, solid, rigid, resistant, impervious) |
| packed(adjective, close, set, hard, firm) |