Same word as irresolvable:
baffling, difficult, impenetrable, indecipherable, inexplicable, inextricable, irresolvable, mystifying, obscure, unaccountable, unconcluded, unfathomable, unresolved, unsolvable, unsolved
baffling, difficult, impenetrable, indecipherable, inexplicable, inextricable, irresolvable, mystifying, obscure, unaccountable, unconcluded, unfathomable, unresolved, unsolvable, unsolved