After an hour, he told them to stop. He himself went down into the hole and then went on digging, but slowly and as though examining the effect of every blow that he struck.
"That's it!" he said, drawing himself up.
"The ground on which we are standing is only a floor of the big buildings that used to adjoin the old keep, buildings which were razed to the ground centuries ago and on the top of which this garden was laid out."
"Well, in clearing away the soil, I have broken through the ceiling of one of the old rooms. Look."
He took a stone, placed it right in the center of the narrower opening which he himself had made and let it drop. The stone disappeared. A dull sound followed almost immediately.
"All that need now be done is for the men to widen the entrance. In the meantime, we will go and fetch a ladder and lights: as much light as possible."
"We have pine torches," said the officer.
Paul was right. When the ladder was let down and he had descended with the lieutenant and Bernard, they saw a very large hall, whose vaults were supported by massive pillars which divided it, like a church of irregular design, into two main naves, with narrower and lower side-aisles.
But Paul at once called his companions' attention to the floor of those two naves:
"A concrete flooring, do you see? . . . And, look there, as I expected, two rails running along one of the upper galleries! . . . And here are two more rails in the other gallery! . . ."
"But what does it all mean?" exclaimed Bernard and the lieutenant.
"It means simply this," said Paul, "that we have before us what is evidently the explanation of the great mystery surrounding the capture of Corvigny and its two forts."
"Corvigny and its two forts were demolished in a few minutes, weren't they? Where did those gunshots come from, considering that Corvigny is fifteen miles from the frontier and that not one of the enemy's guns had crossed the frontier? They came from here, from this underground fortress."
"Here are the rails on which they moved the two gigantic pieces which were responsible for the bombardment."
"I say! You can't bombard from the bottom of a cavern! Where are the embrasures?"
"The rails will take us there. Show a good light, Bernard. Look, here's a platform mounted on a pivot. It's a good size, eh? And here's the other platform."