On the morning of Sunday, the tenth of January, Lieutenant Delroze and Sergeant d'Andeville stepped on to the platform at Corvigny, went to call on the commandant of the town and then took a carriage in which they drove to the ChÂteau d'Ornequin.

"All the same," said Bernard, stretching out his legs in the fly, "I never thought that things would turn out as they have done when I was hit by a splinter of shrapnel between the Yser and the ferryman's house. What a hot corner it was just then! Believe me or believe me not, Paul, if our reinforcements hadn't come up, we should have been done for in another five minutes. We were jolly lucky!"

"We were indeed," said Paul. "I felt that next day, when I woke up in a French ambulance!"

"What I can't get over, though," Bernard continued, "is the way that blackguard of a Major Hermann made off. So you took him prisoner? And then you saw him unfasten his bonds and escape? The cheek of the rascal! You may be sure he got away safe and sound!"

Paul muttered:

"I haven't a doubt of it; and I don't doubt either that he means to carry out his threats against Élisabeth."

"Bosh! We have forty-eight hours before us, as he gave his pal Karl the tenth of January as the date of his arrival and he won't act until two days later."

"And suppose he acts to-day?" said Paul, in a husky voice.

Notwithstanding his anguish, however, the drive did not seem long to him. He was at last approaching—and this time really—the object from which each day of the last four months had removed him to a greater distance. Ornequin was on the frontier; and Èbrecourt was but a few minutes from the frontier. He refused to think of the obstacles which would intervene before he could reach Èbrecourt, discover his wife's retreat and save her. He was alive. Élisabeth was alive. No obstacles existed between him and her.

The ChÂteau d'Ornequin, or rather what remained of it—for even the ruins of the chÂteau had been subjected to a fresh bombardment in November—was serving as a cantonment for territorial troops, whose first line of trenches skirted the frontier. There was not much fighting on this side, because, for tactical reasons, it was not to the enemy's advantage to push too far forward. The defenses were of equal strength; and a very active watch was kept on either side.

These were the particulars which Paul obtained from the territorial lieutenant with whom he lunched.

"My dear fellow," concluded the officer, after Paul had told him the object of his journey, "I am altogether at your service; but, if it's a question of getting from Ornequin to Èbrecourt, you can make up your mind that you won't do it."

"I shall do it all right."

"It'll have to be through the air then," said the officer, with a laugh.

"No."

"Or underground."

"Perhaps."

"There you're wrong. We wanted ourselves to do some sapping and mining. It was no use. We're on a deposit of rock in which it's impossible to dig."

It was Paul's turn to smile:

"My dear chap, if you'll just be kind enough to lend me for one hour four strong men armed with picks and shovels, I shall be at Èbrecourt to-night."

"I say! Four men to dig a six-mile tunnel through the rock in an hour!"

"That's ample. Also, you must promise absolute secrecy both as to the means employed and the rather curious discoveries to which they are bound to lead. I shall make a report to the general commanding in chief; but no one else is to know."

"Very well, I'll select my four fellows for you myself. Where am I to bring them to you?"

"On the terrace, near the donjon."

This terrace commands the Liseron from a height of some hundred and fifty feet and, in consequence of a loop in the river, is exactly opposite Corvigny, whose steeple and the neighboring hills are seen in the distance. Of the castle-keep nothing remains but its enormous base, which is continued by the foundation-walls, mingled with natural rocks, which support the terrace. A garden extends its clumps of laurels and spindle-trees to the parapet.

It was here that Paul went. Time after time he strode up and down the esplanade, leaning over the river and inspecting the blocks that had fallen from the keep under the mantle of ivy.

"Now then," said the lieutenant, on arriving with his men. "Is this your starting-point? I warn you we are standing with our backs to the frontier."

"Pooh!" replied Paul, in the same jesting tone. "All roads lead to Berlin!"

He pointed to a circle which he had marked out with stakes, and set the men to work:

"Go ahead, my lads."

They began to throw up, within a circle of three yards in circumference, a soil consisting of vegetable mold in which, in twenty minutes' time, they had dug a hole five feet deep. Here they came upon a layer of stones cemented together; and their work now became much more difficult, for the cement was of incredible hardness and they were only to break it up by inserting their picks into the cracks. Paul followed the operations with anxious attention.

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.

"What?" asked Bernard.

"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?"

"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.

"That will do capitally."

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."Èbrecourt branch-line," said Paul, laughing. "Seven miles in the shade. And that is how the stronghold of Corvigny was bagged. First, a few thousand men passed through, who killed off the little Ornequin garrison and the posts on the frontier and then went on to the town. At the same time, the two huge guns were brought up, mounted and trained upon sites located beforehand. When these had done their business, they were removed and the holes stopped up. All this didn't take two hours."

"But to achieve those two decisive hours the Kaiser worked for seventeen years, bless him!" said Bernard. "Well, let's make a start."

"Would you like my men to go with you?" suggested the lieutenant.

"No, thank you. It's better that my brother-in-law and I should go by ourselves. If we find, however, that the enemy has destroyed his tunnel, we will come back and ask for help. But it will astonish me if he has. Apart from the fact that he has taken every precaution lest the existence of the tunnel should be discovered, he is likely to have kept it intact in case he himself might want to use it again."

And so, at three o'clock in the afternoon, the two brothers-in-law started on their walk down the imperial tunnel, as Bernard called it. They were well armed, supplied with provisions and ammunition and resolved to pursue the adventure to the end.

In a few minutes, that is to say, two hundred yards farther on, the light of their pocket-lantern showed them the steps of a staircase on their right.

"First turning," remarked Paul. "I take it there must be at least three of them."

"Where does the staircase lead to?"

"To the chÂteau, obviously. And, if you want to know to what part, I say, to the room with the portrait. There's no doubt that this is the way by which Major Hermann entered the chÂteau on the evening of the day when we attacked it. He had his accomplice Karl with him. Seeing our names written on the wall, they stabbed the two men sleeping in the room, Private GÉriflour and his comrade."

Bernard d'Andeville stopped short:

"Look here, Paul, you've been bewildering me all day. You're acting with the most extraordinary insight, going straight to the right place at which to dig, describing all that happened as if you had been there, knowing everything and foreseeing everything. I never suspected you of that particular gift. Have you been studying Sherlock Holmes?"

"Not even ArsÈne Lupin," said Paul, moving on again. "But I've been ill and I have thought things over. Certain passages in Élisabeth's diary, in which she spoke of her perplexing discoveries, gave me the first hint. I began by asking myself why the Germans had taken such pains to create a desert all around the chÂteau. And in this way, putting two and two together, drawing inference after inference, examining the past and the present, remembering my meeting with the German Emperor and a number of things which are all linked together, I ended by coming to the conclusion that there was bound to be a secret communication between the German and the French sides of the frontier, terminating at the exact place from which it was possible to fire on Corvigny. It seemed to me that, a priori, this place must be the terrace; and I became quite sure of it when, just now, I saw on the terrace a dead tree, overgrown with ivy, near which Élisabeth thought that she heard sounds coming from underground. From that moment, I had nothing to do but get to work."

"And your object is ...?" asked Bernard.

"I have only one object: to deliver Élisabeth."

"Your plan?"

"I haven't one. Everything will depend on circumstances; but I am convinced that I am on the right track."

In fact all his surmises were proving to be correct. In ten minutes they reached a space where another tunnel, also supplied with rails, branched off to the right.

"Second turning," said Paul. "Corvigny Road. It was down here that the Germans marched to the town and took our troops by surprise before they even had time to assemble; it was down here that the peasant-woman went who accosted you in the evening. The outlet must be at some distance from the town, perhaps in a farm belonging to the supposed peasant-woman.""And the third turning?" said Bernard.

"Here it is."

"Another staircase?"

"Yes; and I have no doubt that it leads to the chapel. We may safely presume that, on the day when my father was murdered, the Emperor had come to examine the works which he had ordered and which were being executed under the supervision of the woman who accompanied him. The chapel, which at that time was not inside the walls of the park, is evidently one of the exits from the secret network of roads of which we are following the main thoroughfare."

Paul saw two more of these ramifications, which, judging from their position and direction, must issue near the frontier, thus completing a marvelous system of espionage and invasion.

"It's wonderful," said Bernard. "It's admirable. If this isn't Kultur, I should like to know what is. One can see that these people have the true sense of war. The idea of digging for twenty years at a tunnel intended for the possible bombardment of a tiny fortress would never have occurred to a Frenchman. It needs a degree of civilization to which we can't lay claim. Did you ever know such beggars!"

His enthusiasm increased still further when he observed that the roof of the tunnel was supplied with ventilating-shafts. But at last Paul enjoined him to keep silent or to speak in a whisper:

"You can imagine that, as they thought fit to preserve their lines of communication, they must have done something to make them unserviceable to the French. Èbrecourt is not far off. Perhaps there are listening-posts, sentries posted at the right places. These people leave nothing to chance."

One thing that lent weight to Paul's remark was the presence, between the rails, of those cast-iron slabs which covered the chambers of mines laid in advance, so that they could be exploded by electricity. The first was numbered five, the second four; and so on. Paul and Bernard avoided them carefully; and this delayed their progress, for they no longer dared switch on their lamps except at brief intervals.

At about seven o'clock they heard or rather they seemed to hear confused sounds of life and movement on the ground overhead. They felt deeply moved. The soil above them was German soil; and the echo brought the sounds of German life.

"It's curious, you know, that the tunnel isn't better watched and that we have been able to come so far without accident."

"We'll give them a bad mark for that," said Bernard. "Kultur has made a slip."

Meanwhile a brisker draught blew along the walls. The outside air entered in cool gusts; and they suddenly saw a distant light through the darkness. It was stationary. Everything around it seemed still, as though it were one of those fixed signals which are put up near a railway.When they came closer, they perceived that it was the light of an electric arc-lamp, that it was burning inside a shed standing at the exit of the tunnel and its rays were cast upon great white cliffs and upon little mounds of sand and pebbles.

Paul whispered:

"Those are quarries. By placing the entrance to their tunnel there, they were able to continue their works in time of peace without attracting attention. You may be sure that those so-called quarries were worked very discreetly, in a compound to which the workmen were confined."

"What Kultur!" Bernard repeated.

He felt Paul's hand grip his arm. Something had passed in front of the light, like a shadow rising and falling immediately after.

With infinite caution they crawled up to the shed and raised themselves until their eyes were on a level with the windows. Inside were half a dozen soldiers, all lying down, or rather sprawling one across the other, among empty bottles, dirty plates, greasy paper wrappers and remnants of broken victuals. They were the men told off to guard the tunnel; and they were dead-drunk.

"More Kultur," said Bernard.

"We're in luck," said Paul, "and I now understand why the watch is so ill-kept: this is Sunday."

There was a telegraph-apparatus on a table and a telephone on the wall; and Paul saw under a glass case a switch-board with five brass handles, which evidently corresponded by electric wires with the five mine-chambers in the tunnel.

When they passed on, Bernard and Paul continued to follow the rails along the bed of a narrow channel, hollowed out of the rock, which led them to an open space bright with many lights. A whole village lay before them, consisting of barracks inhabited by soldiers whom they saw moving to and fro. They went outside it. They then noticed the sound of a motor-car and the blinding rays of two head-lights; and, after climbing a fence and passing through a shrubbery, they saw a large villa lit up from top to bottom.

The car stopped in front of the doorstep, where some footmen were standing, as well as a guard of soldiers. Two officers and a lady wrapped in furs alighted. When the car turned, the lights revealed a large garden, contained within very high walls.

"It is just as I thought," said Paul. "This forms the counterpart of the ChÂteau d'Ornequin. At either end there are strong walls which allow work to be done unobserved by prying eyes. The terminus is in the open air here, instead of underground, as it is down there; but at least the quarries, the work-yards, the barracks, the garrison, the villa belonging to the staff, the garden, the stables, all this military organization is surrounded by walls and no doubt guarded on the outside by sentries. That explains why one is able to move about so freely inside."At that moment, a second motor-car set down three officers and then joined the other in the coach-house.

"There's a dinner-party on," said Bernard.

They resolved to approach as near as they could, under cover of the thick clumps of shrubs planted along the carriage-drive which surrounded the house.

They waited for some time; and then, from the sound of voices and laughter that came from the ground-floor, at the back, they concluded that this must be the scene of the banquet and that the guests were sitting down to dinner. There were bursts of song, shouts of applause. Outside, nothing stirred. The garden was deserted.

"The place seems quiet," said Paul. "I shall ask you to give me a leg up and to keep hidden yourself."

"You want to climb to the ledge of one of the windows? What about the shutters?"

"I don't expect they're very close. You can see the light shining through the middle."

"Well, but why are you doing it? There is no reason to bother about this house more than any other."

"Yes, there is. You yourself told me that one of the wounded prisoners said Prince Conrad had taken up his quarters in a villa outside Èbrecourt. Now this one, standing in the middle of a sort of entrenched camp and at the entrance to the tunnel, seems to me marked out. ...""Not to mention this really princely dinner-party," said Bernard, laughing. "You're right. Up you go."

They crossed the walk. With Bernard's assistance, Paul was easily able to grip the ledge above the basement floor and to hoist himself to the stone balcony.

"That's it," he said. "Go back to where we were and whistle in case of danger."

After bestriding the balustrade, he carefully loosened one of the shutters by passing first his fingers and then his hand through the intervening space; and he succeeded in unfastening the bolt. The curtains, being crossed inside, enabled him to move about unseen; but they were open at the top, leaving an inverted triangle through which he could see by climbing on to the balustrade.

He did so and then bent forward and looked.

The sight that met his eyes was such and gave him so horrible a blow that his legs began to shake beneath him. ...