Es imposible que hayan podido entrar los enemigos en el edificio porque es impenetrable.There is no way the enemies have been able to break into the building because it is impregnable.
Miguel es impenetrable. No hay forma de entablar una conversación con él porque no habla.Miguel is inscrutable. I just cannot start a conversation with him because he will not talk.
Ningún experto logró descifrar el manuscrito porque estaba escrito en un idioma impenetrable.No expert was able to decipher the manuscript because it was written in an unfathomable language.