inscrutable means mysterious, difficult to penetrate, difficult to solve, impenetrable, inscrutable, incomprehensible.
It can be used as an adjective showing a characteristic of what can not be scrutinized; which can not be investigated or understood; incomprehensible, can not be penetrated because it is impenetrable.
Limbo is where unchristian and pagans go in the after life while the second circle(which i'm assuming you're talking about) is where sinners of Lust go.
