The Spy Paramount by E. Phillips Oppenheim

Fawley, a few nights later, lay on his stomach in the midst of a crumpled heap of undergrowth on almost the topmost spur of the range of mountains eastward from Mont Agel and very little below the snow line. He was on the edge of a recently made clearing and the air was full of the odour of the sawn pine trunks lying about in every direction. The mists rolled over his head and the frozen rain stung his cheeks and pattered against his leather clothes. It was the third moonless night of his almost concluded enterprise and there remained only one unsolved mystery. The six galleries were there, visualised before his eyes. He knew the connecting points of each one and the whereabouts of most of the amazing battery of guns. He knew the entrances and roughly the exits to each. His work had been done with genius and good fortune, yet it was incomplete. The seventh gallery! The key to all the positions. The seventh gallery which must hold the wonder gun. Its exact whereabouts still eluded him.

The night before, the storm which had swept the mountains bare, which had driven even the guards and sentries into shelter, had been a godsend to him. In the roar of the elements and that blinding deluge of rain, the crashing of the trees and the hissing of the wind through the undergrowth and along the ground, he had abandoned caution. He had tramped steadily round from post to post. He had been within a few yards of the Colonel's headquarters. He had even laid his hand on one of the guns, but it was not the gun he sought. He had worked it all out, though, by a process of elimination. The main gallery, the control station and the supreme mystery, which was probably the mightiest anti-aircraft gun in the world, must be somewhere within a radius of about a hundred yards of where he was. It was information invaluable as it stood, but Fawley, through these long hours of darkness and peril, had conceived an almost passionate desire to solve the last enigma of this subterranean mountain fortress. The howling of the wind, which the night before had been his great aid, obscuring all sound and leaving him free to roam about in the darkness in comparative safety, was now, he felt, robbing him of his chance. The special body of men of whom he was in search--he had discovered that many hours ago--worked only in the darkness, so that even the woodcutters should know nothing of their doings. They must be somewhere near now.There were great piles of cement lying within fifty feet of where he was, barrels of mortar, light and heavy trucks. Somewhere close to him they must be working.