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." "How?" "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." "Impossible." "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." "But the embrasures?" "In front of you, Bernard." "That's a wall." "It's the wall which, together with the rock of the hill, supports the terrace above the Liseron, opposite Corvigny. And two circular breaches were made in the wall and afterwards closed up again. You can see the traces of the closing quite plainly." Bernard and the lieutenant could not get over their astonishment: "Why, it's an enormous work!" said the officer. "Absolutely colossal!" replied Paul. "But don't be too much surprised, my dear fellow. It was begun sixteen or seventeen years ago, to my own knowledge. Besides, as I told you, part of the work was already done, because we are in the lower rooms of the old Ornequin buildings; and, having found them, all they had to do was to arrange them according to the object which they had in view. There is something much more astounding, though!" "What is that?" "The tunnel which they had to build in order to bring their two pieces here." "A tunnel?" "Well, of course! How do you expect they got here? Let's follow the rails, in the other direction, and we'll soon come to the tunnel." As he anticipated, the two sets of rails joined a little way back and they saw the yawning entrance to a tunnel about nine feet wide and the same height. It dipped under ground, sloping very gently. The walls were of brick. No damp oozed through the walls; and the ground itself was perfectly dry.