“An impenetrable mystery” was sure “to hang for ever” as far as all mankind was concerned.
Kirkwood smiled grimly, with a face of brass, impenetrable, inflexible.
Go not to the Alhambra: the fort is impenetrable—the guard faithful.
He had been prepared for abuse, but not for impenetrable silence.
Since then Tawabinisy had hidden himself behind his impenetrable grin.