“The Hun for forty years has drafted the insane from every asylum in the Empire to do this gigantic work for him.  Men, women, even children, chained, guarded, have done the physical work....  The Pyramids were builded so, they say....  And in this manner is being finished that colossal engineering work which is never spoken of among the Huns except when necessary, and which is known among them as The Great Secret....  Recklow, it was conceived as a vast engineering project forty-eight years ago—­in 1870 during the Franco-Prussian war.  It was begun that same year....  And it is practically finished.  Except for one obstacle.”

Recklow’s lifted eyes stared at him over his pad.

“It is virtually finished,” repeated McKay in his toneless, unaccented voice which carried such terrible conviction to the other man.  “Forty-eight years ago the Hun planned a huge underground highway carrying four lines of railroad tracks.  It was to begin east of the Rhine in the neighbourhood of Zell, slant into the bowels of the earth, pass deep under the Rhine, deep under the Swiss frontier, deep, deep under Mount Terrible and under the French frontier, and emerge in France behind Belfort, Toul, Nancy, and Verdun.”

Recklow laid his pad on the table and looked intently at McKay.  The latter said in his ghost of a voice:  “You are beginning to suspect my sanity.”  He turned with an effort and fixed his hollow eyes on Evelyn Erith.

“We are sane,” he said.  “But I don’t blame you, Recklow.  We have lived among the mad for more than a year—­among thousands and thousands and thousands of them—­of men and women and even children in whose minds the light of reason had died out....  Thirty thousand dying minds in which only a dreadful twilight reigned!...  I don’t know how we endured it—­and retained our reason....  Do you, Yellow-hair?”

The girl did not reply.  He spoke to her again, then fell silent.  For the girl slept, her delicate, deathly face dropped forward on her breast.

Presently McKay turned to Recklow once more; and Recklow picked up his pad with a slight shudder.

“Forty-eight years,” repeated McKay—­“and the work of the Hun is nearly done—­a wide highway under the earth’s surface flanked by four lines of rails—­broad-gauge tracks—­everything now working, all rolling-stock and electric engines moving smoothly and swiftly....  Two tracks carry troops; two carry ammunition and munitions.  A highway a hundred feet wide runs between.

“Ten miles from the Rhine, under the earth, there is a Hun city, with a garrison of sixty thousand men!...  There are other cities along the line—­”

“Deep down!”

“Deep under the earth.”

“There must be shafts!” said Recklow hoarsely.

“None.”

“No shafts to the surface?”

“Not one.”

“No pipe?  No communication with the outer air?”

Then McKay’s sunken eyes glittered and he stiffened up, and his wasted features seemed to shrink until the parting of his lips showed his teeth.  It was a dreadful laughter—­his manner, now, of expressing mirth.