| firm(adjective, close, set, hard, solid) |
| unintelligible(adjective, knowledge, obscure, solid, dark) |
| invulnerable(adjective, unaffected, unyielding, safety, unbending) |
| mysterious(adjective, obscure, solid, strange, secret) |
| enigmatic(adjective, obscure, difficult, solid, strange) |
| inviolable(adjective, dense, unfathomable, impervious) |
| deep(adjective, set, obscure, solid, heavy) |
| heavy(adjective, set, hard, tough, solid) |
| impregnable(adjective, unyielding, safety) |
| close(adjective, set, hard, tough, solid) |
| inexplicable(adjective, obscure, solid, dark, strange) |
| recondite |
| inaccessible(adjective, hard, unfeeling, unsympathetic, prepared) |
| invincible(adjective, invulnerable) |
| mystic(adjective, obscure, solid, dark, superior) |
| occult(adjective, obscure, dark, superior, strange) |
| tight(adjective, close, solid, short, substantial) |
| condensed(adjective, set, hard, tough, solid) |
| murky(adjective, obscure, solid, dark, gloomy) |
| stony(adjective, hard, firm, stiff, unfeeling) |