References in classic literature ?
They ran thus: "AN IMPENETRABLE MYSTERY SEEMS DESTINED TO HANG FOR EVER OVER THIS ACT OF MADNESS OR DESPAIR.
The same waves wash the moles of the new-built Californian towns, but yesterday planted by the recentest race of men, and lave the faded but still gorgeous skirts of Asiatic lands, older than Abraham; while all between float milky-ways of coral isles, and low-lying, endless, unknown Archipelagoes, and impenetrable Japans.
It recalls a jungle by the confused, varied, and impenetrable aspect of the buildings that line the shore, not according to a planned purpose, but as if sprung up by accident from scattered seeds.
She felt herself clad in an impenetrable armor of falsehood.
He was safe; and on all sides of his dwelling servile fears and servile hopes slept, dreaming of success, behind the severe discretion of doors as impenetrable to the truth within as the granite of tombstones.
By striking numerous matches the Belgian at last found what he sought, and when, a moment later, the sickly rays relieved the Stygian darkness about him, he breathed a nervous sigh of relief, for the impenetrable gloom had accentuated the terrors of his situation.
I want to pay my respects to my dear aunt," Felix answered, perfectly impenetrable to his ungracious reception, and perfectly comfortable in a spacious arm-chair.
 "And how many impenetrable battleships strike terror to the hearts
They had to raise enormous stones, massive pieces of wrought iron, heavy corner-clamps and huge portions of cylinder, with an object-glass weighing nearly 30,000 pounds, above the line of perpetual snow for more than 10,000 feet in height, after crossing desert prairies, impenetrable forests, fearful rapids, far from all centers of population, and in the midst of savage regions, in which every detail of life becomes an almost insoluble problem.
He had a haughty bearing, a look either steady and impenetrable or insolently piercing and inquisitorial.
Meantime bands of Convicts occupied every entrance with an impenetrable phalanx.
Let your plans be dark and impenetrable as night, and when you move, fall like a thunderbolt.