Adjective : an impenetrable mystery.    From Dictionary.com. 

I had a tough time reading through your impenetrableness.    From Wordnik.com.  [High on Christology] Reference

This impenetrableness, my dear, is to be put among the shades in his character.    From Wordnik.com.  [Clarissa Harlowe] Reference

I had no knowledge of this art; but neither custom, nor law, nor the impenetrableness of the mystery, required me to serve a seven years 'apprenticeship to it.    From Wordnik.com.  [Arthur Mervyn Or, Memoirs of the Year 1793] Reference

The shades of October were far advanced, and she was still a prey to the cruel fluctuations of hope and disappointment that the uncertainty and impenetrableness of Mr. Willoughby subjected her to.    From Wordnik.com.  [Isabella. A Novel] Reference

His appearance and general behaviour might have strongly interested all persons in his favour; but the coldness of his address, and the impenetrableness of his sentiments, seemed to forbid those demonstrations of kindness to which one might otherwise have been prompted.    From Wordnik.com.  [Caleb Williams Or Things as They Are] Reference

He did not know why it was, but the beetling crags above him, the consciousness of the marvellous plains below, the rhythmic murmur of the wind in the pine trees near at hand, the curious impenetrableness of the old earth, the kingship of death asserting itself in the motionless brute which he had killed, but which he was powerless to make alive again -- all these weird and unaccustomed influences seemed to be clutching at his imagination, taking liberties with his sense of identity.    From Wordnik.com.  [Peak and Prairie From a Colorado Sketch-book] Reference

There was an impenetrableness in the reserve.    From Wordnik.com.  [Stubble] Reference

It was discovered that thousands of square miles of Australian soil never catch glimpses of the sun in consequence of the impenetrableness of the shade of Australian trees; that the scent of the wattles, the eucalypts, the boronias, the hoyas, the gardenias, the lotus, etc., etc., are among the sweetest and cleanest, most powerful and most varied in the world; that many of the birds of Australia have songs full of melody; that the so-called Australian cherry is no more a cherry than an acorn; that the Australian dog (though “the only true wild dog in the world”) is deemed to be a comparatively recent introduction — a new chum of Asiatic origin who entered the glorious constellation of the State something before the era of exclusive legislation — so naturally he does not bark, for barking is an evidence of civilisation; but he soon learns the universal language of the dog.    From Wordnik.com.  [The Confessions of a Beachcomber] Reference