As they crouched there, Frank’s voice was taking on a frightened tone and Tom could now hear it much more plainly. But Tom’s mind was filled with the danger of being discovered and he scarcely dared reply, for somehow, although there was no foundation for his fears, he was filled with a terrible dread of these under-sea workers, these unknown mysterious divers who had lifted the ominous-looking metal cylinder through the trapdoor in the disused sewer. That, even if they heard him and his friends, they could not trail them or locate them under water never occurred to him. In fact, he had quite forgotten that he and Rawlins were under water or were as invisible to others a few yards away as other objects were to them. He felt as though he could be as easily seen as if on land, and that, if he spoke, his words would at once betray his whereabouts. But he also realized that Frank’s voice could be heard by others as well as by himself and so, steeling himself to the effort, he called back, “It’s all right. Don’t worry. Stop talking and listen.”

Instantly, Frank’s voice ceased and Tom drew a breath of relief and then he gulped and pressed close to Rawlins, for before him in the water, as if attracted by the sounds of his voice, the two dim forms of the strange divers once more appeared. For a space they remained motionless as though listening and perspiration broke out on Tom’s forehead and chills ran up and down his spine as he heard distinctly the sounds of low-toned words in the same guttural tongue, and he was certain, positive, that his voice had been heard, that the others were striving to locate him, and that at any moment Frank or Henry might become curious or impatient and speak. In his terrified mind he could picture those big, sickly green, distorted beings creeping towards him, their wide-flung arms waving uncertainly like the tentacles of a huge octopus as they lurched forward; he could imagine the fixed, expressionless stare of those great goggle-eyed glasses in the squat neckless helmets and, as the current caused light and shadow to waver and change, the figures seemed actually moving towards his hiding place.

It was terrible; he no longer thought of them as fellow men, no longer looked upon them as human beings; his fears had transformed them to submarine monsters, weird, uncanny, intelligent but bloodthirsty creatures, and so great was the tension, so fearful the vision conjured up by his overwrought imagination that he would have screamed had his mouth not been parched and dry and incapable of uttering a sound. It was like a nightmare, a dream in which one is powerless to move or to cry out; where one cannot compel muscles or mind to function; where one feels that it cannot be real, cannot be possible and yet is filled with sweating, blood-curdling terror that it is. And then, after what seemed hours of torture, but was barely ten seconds from the time the men had emerged from the sewer, their voices ceased and to Tom’s inexpressible relief they appeared to fade into the murky green water. They were moving away, soundlessly, mysteriously, without visible effort, and Tom noticed with his fright-filled eyes that above them was poised an indistinct, cigar-shaped object, the same torpedolike affair he had seen lifted from the sewer, and he realized that somehow, by some means, the men were dragging this along with them into the dim, green distance.

He was aroused by Rawlins’ whisper and a touch on his arm.

“I’m going to follow,” were the barely audible words. “No danger. Must see where they go. Come on.”

Recovering from his fright, now that the divers were retreating, and rather reassured by the sound of his companion’s voice and words Tom moved forward from the wall but still grasping Rawlins’ hand.

They could not see the figures before them, for the muck stirred up by the others’ passage concealed them as effectually as a smoke screen, but it also served to betray their whereabouts and to conceal Tom and Rawlins as well. For some distance—several hundred yards Tom thought—they moved along, following close to the wall that bounded the shore and ever with the slowly drifting column of muddy water to guide them.

Once or twice the murk seemed to drift away, and each time Rawlins instantly halted, waiting to see if those they were trailing had come to a stop, but each time the mud again rose before them and they resumed their way.

Tom had no idea of the distance or direction they had traveled. The effort he made to walk was his only guide and he knew that the same effort, the same number of steps on land, would have carried him a long way, but he also knew that under water his progress was snail-like, that a step might carry one a few inches or several feet or not at all, depending upon the current, and he wondered vaguely if Rawlins knew his way, if he could find his way back, or if he intended to bob to the surface to get his bearings when he finally decided to return to the dock. And Tom smiled to himself as he pictured the looks of surprise, the screams of fright which would greet their unexpected and sudden appearance if Rawlins did this and they should bob up beside some crowded recreation pier or ferry ship. But Rawlins had halted again.

Before them now the mud was thinning out, the water was being swept clear of silt and Rawlins drew Tom beside him behind a huge block of stone which had been dumped at the base of the wall. Slowly and gradually the water cleared. It was evident that those they were following were no longer stirring up the mud and so must have come to a stop and, as the sediment drifted off and the dim green light filtered through the water, Tom peered into the vast illimitable void. It was like looking through thick green glass or like glass made half-opaque by one’s breath upon it and for a time Tom could see nothing. Then, as the water became still clearer, he saw the faint outlines of timbers and spiles and a dark object looming ominously, like a cloud, which he recognized as the bottom of some vessel. Against the lighter water over his head, a shadow passed and the greenness quivered and wavered and he knew a small boat was being rowed above them; but no sign could he see of those they had been following.

Then Tom noticed something else, something that rose above the dark bottom of the river as a darker mass, something that resembled a great bank of mud or a reef of rock. Irregular in outline, dark green as seen through the water, unlike anything he had ever seen, yet somehow it had a vaguely familiar look; it did not seem quite like mud or rock of any natural formation, but rather like some sort of boat. Yes, that was it—like the hull of a boat—it reminded him of a picture of a sunken wreck. Perhaps it was. Yes, now that the thought had entered his head, he could see that it was a wreck; he could make out the stump of a mast, the remains of deck houses, something like portions of rails. But what was it doing here? Why should a sunken hulk be lying in the East River? Of course it was out of the channel, it was lying partly beneath a dock or pier and Tom noticed that the spiles of the pier sagged and that several were broken off under water. Evidently the pier was an old one, perhaps disused, and maybe the old hulk had been sunk during some fire which had destroyed the pier at the same time.

All these thoughts flashed through Tom’s mind as he peered into the dim greenness and then all were wiped from his brain as he caught a glimpse of the two divers moving from among the spiles. Tom was as much at sea as ever as to the distances under water. He could not tell whether the wreck was fifty or five hundred feet away. He was not at all sure that, if he reached out, he could not touch the old hulk or even the moving forms. The next moment the two had reached the side of the wreck and then, to Tom’s amazement, they seemed to disappear within it, to step through the sides as though it were only a shadow in the water.

“Gosh,” he ejaculated unconsciously, “they went into that wreck!”

“Wreck!” came Rawlins’ whispered words. “Wreck! That’s no wreck. That’s a submarine. That’s their hangout!”

So absolutely thunderstruck was Tom at Rawlins’ words that he could not even reply. But now he saw that what he had mistaken for a waterlogged sunken hulk was indeed an under-sea boat, a submarine and a big one. He had never seen a submarine except from above water before. He had no idea how such a craft would appear under water. He did not realize that the narrow deck almost awash, the tiny superstructure and conning tower which are all the landsman sees are but a very small portion of a submarine’s whole; that out of sight, and never exposed above the surface of the sea, is a big boat-shaped hull with rudders and propellers; that the cigar-shaped Jules Verne type of submersible so familiar in fiction is not a thing of fact; and that the modern submarine if seen under water might easily be mistaken for an ordinary vessel’s hull.

It was not at all surprising therefore that Tom had mistaken the submerged craft for the hulk of a steamer or ship, for submarines were the last thing in his mind and no one would have dreamed of seeing one here beneath the surface of the East River.

Now, however, Tom could see that what he had mistaken for the stump of a mast was the conning tower; what he had thought were shattered deck houses and rails were the superstructure; and he could now even make out the lateral horizontal rudders and the vertical rudder and screws.

But this made the mystery still greater. It was even more wonderful to find a submarine here than a sunken vessel. Of course, Tom knew there were plenty of the navy’s submarines forever knocking about, and for an instant it occurred to him that it was one of these engaged in making some test and that the divers whom they had seen were members of the boat’s crew.

Then instantly he remembered the men had spoken in a foreign tongue, that they had carried a mysterious object to the trapdoor in the sewer, and that they had taken the same or a duplicate object from the sewer.

It was all inexplicable, puzzling, unfathomable.

Rawlins’ voice recalled him to the present.

“They’ve gone,” said the diver. “I want to find out who and what she is. You stay here. I’ll be away only a moment.”

As he spoke, he released Tom’s hand and with a final caution for the boy not to follow or move away, Rawlins floundered towards the submarine.

Interestedly Tom watched him. He noticed that Rawlins did not stir up the mud and then, for the first time, he discovered that the bottom was hard and sandy. Somehow all sense of fear and danger had left him. How foolish he had been, to be sure! No doubt, he thought to himself, it was the unexpected appearance of the men and their grotesque forms which had aroused his imagination. There was Rawlins, still moving away and looking as terrible and awesome as had the others—even more so, if anything, with his proboscis-like helmet topped by its grid and the container on his shoulders giving him the appearance of being humpbacked.

He wondered how far the submarine was from where he stood. Rawlins now seemed close to it and yet he could not possibly tell whether his friend was really near to the craft or not. It was all most interesting, most baffling and most unreal and dreamlike. He wondered what Frank and Henry would think of his long silence. He wondered if they could hear him or he could hear them. Surely there would be no danger in speaking now. Even if those in the submarine heard him they could not tell whether it was some one under or above the water who was speaking. Why hadn’t he thought of that before? There never had been any danger. Of course, if these men had under-sea radio they must hear messages from those on land as well as the boys.

In that case they would never have had suspicions if they had overheard the boys’ conversation. They would never dream that others possessed the apparatus and would have assumed that the speakers were on shore. There was no danger; he was sure of it, and he was about to call to Frank when his attention was arrested by Rawlins’ actions.

Tom had been idly watching him and had seen him reach the submarine. He had seen Rawlins moving around the craft, evidently examining it, and he had lost sight of him as Rawlins had slipped around the blunt bow. But now Rawlins suddenly appeared, backing into view, waving his arms to maintain his balance and floundering. And he held something in one hand, something that he waved menacingly above his head, some object that glittered even in the dull, subdued, green light.

For the space of a second, Tom was puzzled and then he knew. It was Rawlins’ hatchet! Something or some one was attacking him and scarcely had this knowledge flashed through Tom’s mind when, from behind the submarine, the two figures appeared, clutching arms pawing at the water as if swimming, bodies bent far forward, their every attitude, every motion betokening speed, speaking of straining efforts to come within reach of Rawlins, despite his threatening, keen-edged hatchet.

Wildly excited, filled with deadly fear, terrorized at Rawlins’ plight as was Tom, yet through his mind ran the thought, the subconscious feeling, that it was all unreal—a dream or a delusion. It was unspeakably and inexpressibly uncanny to see the three men evidently exerting every effort and yet moving so silently and slowly, seeming to float like weightless bodies in some semi-transparent, green medium. It reminded Tom of a slow motion picture—one of the films where a man or a horse, leaping a hurdle, appears to float lightly as a bit of thistledown through the air—and watching, the boy was fascinated. But only for the briefest moment.

Scarcely had the three come within Tom’s view when Rawlins stumbled over an upjutting stub of spiling, the hatchet flew from his hand and before he could half rise the others were upon him.

At this, the spell was broken. Tom screamed aloud and the next instant, like a voice from another sphere, he heard Frank speaking.

“What is the matter, Tom? What’s wrong?” came in troubled, worried tones. “Why did you yell?”

Here then was help. They were still within reach of those ashore and in terse, excited, fear-wrung tones Tom answered.

“Help! Send for help!” he yelled, entirely forgetting that no one knew where he was or where to send help even if help could have reached them there under the river.

“It’s awful!” he continued. “Two men—divers—from a submarine—fighting with Mr. Rawlins! They’re attacking him—struggling with him! Get Dad, get the police!”

Then, faint and as from a vast distance, he heard Frank’s voice calling excitedly for Mr. Pauling’s telephone number. He knew his chum was summoning aid and he sat rigid, watching with staring eyes the struggle taking place beneath the river. Rawlins had arisen; by a tremendous effort he had flung aside one man, but the other was grappling with him, fighting desperately, and as Tom saw something flash in the water above the struggling men’s heads he realized that the stranger held a knife.

Now they had drawn closer, they were some distance from the submarine and the very instant Tom noticed this a wild cry of alarm rang in his ears.

At the sound, Tom saw one man start to plunge towards the under-sea boat, and to the boy’s astonishment he saw that the craft was moving and was slipping rapidly from its resting place. Although the man struggled desperately to reach it he might as well have stood still, for scarcely did Tom realize that the submarine was under way ere it was a mere shadow and a second later had faded into the murky green.

And now Tom saw that Rawlins was the aggressor, the man who had been chasing the submarine was swaying drunkenly, whirling in a half-circle, his arms waving helplessly, while his companion had broken away from Rawlins and was standing, with hands upraised, and backing slowly away from the latter who leaned towards him with the other’s knife in his hand.

“Kamarad!” Tom heard in thick tones. “Kamarad!” and the boy’s heart jumped as he heard the words of surrender, the words which had become so familiar to thousands of men in the trenches, and Tom, with a shock of surprise, realized that the divers were Germans.

Now he could hear Rawlins’ words, spoken as if to himself or as if he thought the others could hear.

“Yes, you dirty skunk!” Tom caught. “I’ll tell the world you’ll surrender. All right, right about face and forward march and no nonsense or I’ll puncture that suit and your hide under it.”

And then Tom’s brain had another sudden jolt. Of course the German could hear. Of course Rawlins had heard his cry of surrender. What a dolt he had been! They had radio sets, they could hear everything that was said as readily as he could. That was why they had given up the fight, yes that was it, that was why the submarine had cleared out. They had heard his cry for help, had heard him tell Frank to summon police. How could they know that their whereabouts was not known, that it was mere chance that he and Rawlins had stumbled upon them? No doubt they imagined they had been watched, trailed and surrounded and the submarine, rather than run the risk of being captured, had deserted the two men at the first sound of alarm being given. It was all clear to Tom now. The battle was over. Rawlins was victorious, the men were his prisoners. Now Rawlins was speaking again and Tom saw that the second man was being half-dragged along by his fellow. But Rawlins’ words aroused Tom to instant activity.

“Are you all right, Tom?” asked Rawlins. “Come over here. We need a hand. This chap’s hurt somehow. Can’t get an answer out of him. Short circuited or something. We’ve got to get him out somehow.”

Lunging forward, Tom bumped into Rawlins before he had taken six steps and gave a startled exclamation. Was it possible the fight had taken place so close? But he had no time to think on this matter. The second man was helpless, dead, as far as appearances went, and Rawlins, stooping quickly, cut the lead-soled boots from his feet.

Thus relieved of the weights, the body partly floated and with Tom holding to one arm and the captured man grasping the other, while Rawlins kept a hand on Tom and directed the way, the strange under-sea procession floundered through the water, along the wall, past the black sewer mouth and towards Rawlins’ dock.

And now Tom again heard Frank’s voice.

“Where are you?” it asked. “Your father’s coming. How can they find you? Are you all right?”

“Everything’s all right,” answered Tom. “We’re coming back. Be there soon!”

Hardly a minute later, Tom saw the familiar piers near their own dock. He had thought they had wandered far, but they had not been two hundred yards distant at any time. A moment later, they reached the foot of the ladder.

Telling Tom to go up, Rawlins half lifted the unconscious man and with a gruff warning to his fellow started to mount the rungs. Evidently the words were heard by the anxious, waiting boys above, for Tom heard Frank’s shout of joy and he called back as he drew himself towards the open trap.

But before his head emerged from the water, a crash like thunder sounded in his ears, there was a sound of tramping, hurrying footsteps, shouts and cries and Tom’s brain reeled. What was happening? Had the men’s confederates learned of their capture? Were their fellows breaking into the laboratory to rescue them? Were the ruffians wreaking vengeance on Frank and Henry?