WordNet sense 1 (of an obscure nature):

cryptic, inscrutable, mysterious

WordNet sense 2 (incapable of being explained or accounted for):

inexplicable

WordNet sense 3 (not clearly understood or expressed):

opaque, unintelligible

WordNet sense 4 (impossible to understand):

impenetrable

WordNet sense 5 (impossible to come to understand):

unfathomable

WordNet sense 6 (not clear to the understanding):

enigmatic

WordNet sense 7 (marked by difficulty of style or expression):

obscure

WordNet sense 8 (difficult to understand):

uncomprehensible