Dr. Bird looked up impatiently as the door of his private laboratory in the Bureau of Standards swung open, but the frown on his face changed to a smile as he saw the form of Operative Carnes of the United States Secret Service framed in the doorway.

“Hello, Carnes,” he called cheerfully. “Take a seat and make yourself at home for a few minutes. I’ll be with you as soon as I finish getting this weight.”

Carnes sat on the edge of a bench and watched with admiration the long nervous hands and the slim tapering fingers of the famous scientist. Dr. Bird stood well over six feet and weighed two hundred and six pounds stripped: his massive shoulders and heavy shock of unruly black hair combined to give him the appearance of a prize-fighter--until one looked at his hands. Acid stains and scars could not hide the beauty of those mobile hands, the hands of an artist and a dreamer. An artist Dr. Bird was, albeit his artistry expressed itself in the most delicate and complicated experiments in the realms of pure and applied science that the world has ever seen, rather than in the commoner forms of art.

The doctor finished his task of weighing a porcelain crucible, set it carefully into a dessicator, and turned to his friend.

“What’s on your mind, Carnes?” he asked. “You look worried. Is there another counterfeit on the market?”

The operative shook his head.

“Have you been reading those stories that the papers have been carrying about Mammoth Cave?” he asked.

Dr. Bird emitted a snort of disgust.

“I read the first one of them part way through on the strength of its being an Associated Press dispatch,” he replied, “but that was enough. It didn’t exactly impress me with its veracity, and, from a viewpoint of literature, the thing was impossible. I have no time to pore over the lucubrations of an inspired press agent.”

“So you dismissed them as mere press agent work?”

“Certainly. What else could they be? Things like that don’t happen fortuitously just as the tourist season is about to open. I suppose that those yarns will bring flocks of the curious to Kentucky though: the public always responds well to sea serpent yarns.”

“Mammoth Cave has been closed to visitors for the season,” said Carnes quietly.

“What?” cried the doctor in surprise. “Was there really something to those wild yarns?”

“There was, and what is more to the point, there still is. At least there is enough to it that I am leaving for Kentucky this evening, and I came here for the express purpose of asking you whether you wanted to come along. Bolton suggested that I ask you: he said that the whole thing sounded to him like magic and that magic was more in your line than in ours. He made out a request for your services and I have it in my pocket now. Are you interested?”

“How does the secret service cut in on it?” asked the doctor. “It seems to me that it is a state matter. Mammoth Cave isn’t a National Park.”

“Apparently you haven’t followed the papers. It was a state matter until the Governor asked for federal troops. Whenever the regulars get into trouble, the federal government is rather apt to take a hand.”

“I didn’t know that regulars had been sent there. Tell me about the case.”

“Will you come along?”

Dr. Bird shook his head slowly.

“I really don’t see how I can spare the time, Carnes,” he said. “I am in the midst of some work of the utmost importance and it hasn’t reached the stage where I can turn it over to an assistant.”

“Then I won’t bother you with the details,” replied Carnes as he rose.

“Sit down, confound you!” cried the doctor. “You know better than to try to pull that on me. Tell me your case, and then I’ll tell you whether I’ll go or not. I can’t spare the time, but, on the other hand, if it sounds interesting enough...”

Carnes laughed.

“All right, Doctor,” he said, “I’ll take enough time to tell you about it even if you can’t go. Do you know anything about it?”

“No. I read the first story half way through and then stopped. Start at the beginning and tell me the whole thing.”

“Have you ever been to Mammoth Cave?”

“No.”

“It, or rather they, for while it is called Mammoth Cave it is really a series of caves, are located in Edmonson County in Central Kentucky, on a spur railroad from Glasgow Junction on the Louisville and Nashville Railroad. They are natural limestone caverns with the customary stalactite and stalagmite formation, but are unusually large and very beautiful. The caves are quite extensive and they are on different levels, so that a guide is necessary if one wants to enter them and be at all sure of finding the way out. Visitors are taken over a regular route and are seldom allowed to visit portions of the cave off these routes. Large parts of the cave have never been thoroughly explored or mapped. So much for the scene.

“About a month ago a party from Philadelphia who were motoring through Kentucky, entered the cave with a regular guide. The party consisted of a man and his wife and their two children, a boy of fourteen and a girl of twelve. They went quite a distance back into the caves and then, as the mother was feeling tired, she and her husband sat down, intending to wait until the guide showed the children some sights which lay just ahead and then return to them. The guide and the children never returned.”

“What happened?”

“No one knows. All that is known is the bare fact that they have not been seen since.”

“A kidnapping case?”

“Apparently not, in the light of later happenings, although that was at first thought to be the explanation. The parents waited for some time. The mother says that she heard faint screams in the distance some ten minutes after the guide and the children left, but they were very far away and she isn’t sure that she heard them at all. At any rate, they didn’t impress her at the time.

“When half an hour had passed they began to feel anxious, and the father took a torch and started out to hunt for them. The usual thing happened; he got lost. When he failed to return, the mother, now thoroughly alarmed, made her way, by some uncanny sense of direction, to the entrance and gave the alarm. In half an hour a dozen search parties were on their way into the cave. The father was soon located, not far from the beaten trail, but despite three days of constant search, the children were not located. The only trace of them that was found was a bracelet which the mother identified. It was found in the cavern some distance from the beaten path and was broken, as though by violence. There were no other signs of a struggle.

“When the bracelet was found, the kidnapping theory gained vogue, for John Harrel, the missing guide, knew the cave well and natives of the vicinity scouted the idea that he might be lost. Inspired by the large reward offered by the father, fresh parties began to explore the unknown portions of the cave. And then came the second tragedy. Two of the searchers failed to return. This time there seemed to be little doubt of violence, for screams and a pistol shot were faintly heard by other searchers, together with a peculiar ‘screaming howl, ‘ as it was described by those who heard it. A search was at once made toward the spot where the bracelet had been picked up, and the gun of one of the missing men was found within fifty yards of the spot where the bracelet had been discovered. One cylinder of the revolver had been discharged.”

“Were there any signs on the floor?”

“The searchers said that the floor appeared to be rather more moist and slimy than usual, but that was all. They also spoke of a very faint smell of musk, but this observation was not confirmed by others who arrived a few moments later.”

“What happened next?”

“The Governor was appealed to and a company of the National Guard was sent from Louisville to Mammoth Cave. They took up camp at the mouth of the cave and prevented everyone from entering. Soldiers armed with service rifles penetrated the caverns, but found nothing. Visitors were excluded, and the guardsmen established regular patrols and sentry posts in the cave with the result that one night, when time came for a relief, the only trace that could be found of one of the guards was his rifle. It had not been fired. Double guards were then posted, and nothing happened for several days--and then another sentry disappeared. His companion came rushing out of the cave screaming. When he recovered, he admitted that both he and the missing man had gone to sleep and that he awoke to find his comrade gone. He called, and he says that the answer he received was a peculiar whistling noise which raised all the hair on the back of his neck. He flashed his electric torch all around, but could see nothing. He swears, however, that he heard a slipping, sliding noise approaching him, and he felt that some one was looking at him. He stood it as long as he could and then threw down his rifle and ran for his life.”

“Had he been drinking?”

“No. It wasn’t delirium either, as was shown by the fact that a patrol found his gun where he had thrown it, but no trace of the other sentry. After this second experience, the guardsmen weren’t very eager to enter the cave, and the Governor asked for regulars. A company of infantry was ordered down from Fort Thomas to relieve the guardsmen, but they fared worse than their predecessors. They lost two men the first night of their guard. The regulars weren’t caught napping, for the main guard heard five shots fired. They rushed a patrol to the scene and found both of the rifles which had been fired, but the men were gone.

“The officer of the day made a thorough search of the vicinity and found, some two hundred yards from the spot where the sentries had been posted, a crack in the wall through which the body of a man could be forced. This bodycrack had fresh blood on each side of it. Several of his men volunteered to enter the hole and search, but the lieutenant would not allow it. Instead, he armed himself with a couple of hand-grenades and an electric torch and entered himself. That was last Tuesday, and he has not returned.”

“Was there any disturbance heard from the crack?”

“None at all. A guard was posted with two machine-guns pointed at the crack in the wall, and a guard of eight men and a sergeant stationed there. Last night, about six o’clock, while the guard were sitting around their guns, a faint smell of musk became evident. No one paid a great deal of attention to it, but suddenly for no apparent reason at all one of the men on guard was jerked into the air feet upwards. He gave a scream of fear, and an unearthly screech answered him. The guard, with the exception of one man, turned tail and ran. One man stuck by his gun and poured a stream of bullets into the crack. The retreating men could hear the rattle of the gun for a few moments and then there was a choking scream, followed by silence. When the officer of the day got back with a patrol, there was a heavy smell of musk in the air, and a good deal of blood was splashed around. The machine-guns were both there, although one of them was twisted up until it looked like it had been through an explosion.

“The Officer commanding the company investigated the place, ordered all men out of the cave, and communicated with the War Department. The Secretary of War found it too tough a nut to crack and he asked for help, so Bolton is sending me down there. Do you think, in view of this yarn, that your experiments can wait?”

The creases on Dr. Bird’s high forehead had grown deeper and deeper as Carnes had told his story, but now they suddenly disappeared, and he jumped to his feet with a boyish grin.

“How soon are we leaving?” he asked.

“In two hours, Doctor. A car is waiting for us downstairs and I have reservations booked for both of us on the Southern to-night. I knew that you were coming; in fact, the request for your services had been approved before I came here to see you.”

Dr. Bird rapidly divested himself of his laboratory smock and took his coat and hat from a cupboard.

“I hope you realize, Carnsey, old dear,” he said as he followed the operative out of the building, “that I have a real fondness for your worthless old carcass. I am leaving the results of two weeks of patient work alone and unattended in order to keep you out of trouble, and I know that it will be ruined when I get back. I wonder whether you are worth it?”

“Bosh!” retorted Carnes. “I’m mighty glad to have you along, but you needn’t rub it in by pretending that it is affection for me that is dragging you reluctantly into this mess. With an adventure like this ahead of you, leg-irons and handcuffs wouldn’t keep you away from Mammoth Cave, whether I was going or not.”

It was late afternoon before Dr. Bird and Carnes dismounted from the special train which had carried them from Glasgow Junction to Mammoth Cave. They introduced themselves to the major commanding the guard battalion which had been ordered down to reinforce the single company which had borne the first brunt of the affair, and then interviewed the guards who had been routed by the unseen horror which was haunting the famous cave. Nothing was learned which differed in any great degree from the tale which Carnes had related to the doctor in Washington, except that the officer of the day who had investigated the last attack failed to entirely corroborate the smell of musk which had been reported by the other observers.

“It might have been musk, but to me it smelled differently,” he said. “Were you ever near a rattlesnake den in the west?”

Dr. Bird nodded.

“Then you know the peculiar reptilian odor which such a place gives off. Well, this smell was somewhat similar, although not the same by any manner of means. It was musky all right, but it was more snake than musk to me. I rather like musk, but this smell gave me the horrors.”

“Did you hear any noises?”

“None at all. The men describe some rather peculiar noises and Sergeant Jervis is an old file and pretty apt to get things straight, but they may have been made by the men who were in trouble. I saw a man caught by a boa in South America once, and the noises he made might very well have been described in almost the same words as Jervis used.”

“Thanks, Lieutenant,” replied the Doctor. “I’ll remember what you have told me. Now I think that we’ll go into the cave.”

“My orders are to allow no one to enter, Doctor.”

“I beg your pardon. Carnes, where is that letter from the Secretary of War?”

Carnes produced the document. The lieutenant examined it and excused himself. He returned in a few moments with the commanding officer.

“In the face of that letter, Dr. Bird,” said the major, “I have no alternative to allowing you to enter the cave, but I will warn you that it is at your own peril. I’ll give you an escort, if you wish.”

“If Lieutenant Pearce will come with me as a guide, that will be all that I need.”

The lieutenant paled slightly, but threw back his shoulders.

“Do you wish to start at once, sir?” he asked.

“In a few moments. What is the floor of the cave like where we are going?”

“Quite wet and slimy, sir.”

“Very slippery?”

“Yes, sir.”

“In that case before we go in we want to put on baseball shoes with cleats on them, so that we can run if we have to. Can you get us anything like that?”

“In a few moments, sir.”

“Good! As soon as we can get them we’ll start. In the meantime, may I look at that gun that was found?”

The Browning machine-gun was laid before the doctor. He looked it over critically and sniffed delicately at it. He took from his pocket a phial of liquid, moistened a portion of the water-jacket of the weapon, and then rubbed the moistened part briskly with his hand. He sniffed again. He looked disappointed, and again examined the gun closely.

“Carnes,” he said at length, “do you see anything on this gun that looks like tooth marks?”

“Nothing, Doctor.”

“Neither do I. There are some marks here which might quite conceivably be finger-prints of a forty-foot giant, and those two parallel grooves look like the result of severe squeezing, but there are no tooth marks. Strange. There is no persistent odor on the gun, which is also strange. Well, there’s no use in theorizing: we are confronted by a condition and not a theory, as someone once said. Let’s put on those baseball shoes and see what we can find out.”

Dr. Bird led the way into the cave, Carnes and the lieutenant following closely with electric torches. In each hand Dr. Bird carried a phosphorus hand-grenade. No other weapons were visible, although the doctor knew that Carnes carried a caliber .45 automatic pistol strapped under his left armpit. As they passed into the cave the lieutenant stepped forward to lead the way.

“I’m going first,” said the doctor. “Follow me and indicate the turns by pressure on my shoulder. Don’t speak after we have started, and be ready for instant flight. Let’s go.”

Forward into the interior of the cave they made their way. The iron cleats of the baseball shoes rang on the floor and the noise echoed back and forth between the walls, dying out in little eerie whispers of sound that made Carnes’ hair rise. Ever forward they pressed, the lieutenant guiding the doctor by silent pressure on his shoulder and Carnes following closely. For half a mile they went on until a restrainable pressure brought the doctor to a halt. The lieutenant pointed silently toward a crack in the wall before them. Carnes started forward to examine it, but a warning gesture from the doctor stopped him.

Slowly, an inch at a time, the doctor crept forward, hand-grenades in readiness. Presently he reached the crack and, shifting one of the grenades into his pocket, he drew forth an electric torch and sent a beam of light through the crack into the dark interior of the earth.

For a moment he stood thus, and then suddenly snapped off his torch and straightened up in an attitude of listening. The straining ears of Carnes and Lieutenant Pearce could hear a faint slithering noise coming toward them, not from the direction of the crack, but from the interior of the cave. Simultaneously a faint, musky, reptilian odor became apparent.

“Run!” shouted the doctor. “Run like hell! It’s loose in the cave!”

The lieutenant turned and fled at top speed toward the distant entrance to the cave, Carnes at his heels. Dr. Bird paused for an instant, straining his ears, and then threw a grenade. A blinding flash came from the point where the missile struck and a white cloud rose in the air. The doctor turned and fled after his companions. Not for nothing had Dr. Bird been an athlete of note in his college days. Despite the best efforts of his companions, who were literally running for their lives, he soon caught up with them. As he did so a weird, blood-curdling screech rose from the darkness behind them. Higher and higher in pitch the note rose until it ended suddenly in a gurgling grunt, as though the breath which uttered it had been suddenly cut off. The slithering, rustling noise became louder on their trail.

“Faster!” gasped the doctor, as he put his hand on Carnes’ shoulder and pushed him forward.

The noise of pursuit gained slightly on them, and a sound as of intense breathing became audible. Dr. Bird paused and turned and faced the oncoming horror. His electric torch revealed nothing, but he listened for a moment, and then threw his second grenade. Keenly he watched its flight. It flew through the air for thirty yards and then struck an invisible obstruction and bounded toward the ground. Before it struck the downward motion ceased, and it rose in the air. As it rose it burst with a sharp report, and a wild scream of pain filled the cavern with a deafening roar. The doctor fled again after his companions.

By the time he overtook them the entrance of the cave loomed before them. With sobs of relief they burst out into the open. The guards sprang forward with raised rifles, but Dr. Bird waved them back.

“There’s nothing after us, men,” he panted. “We got chased a little way, but I tossed our pursuer a handful of phosphorus and it must have burned his fingers a little, judging from the racket he made. At any rate, it stopped the pursuit.”

The major hurried up.

“Did you see it, Doctor?” he asked.

“No, I didn’t. No one has ever seen it or anything like it. I heard it and, from its voice, I think it has a bad cold. At least, it sounded hoarse, so I gave it a little white phosphorus to make a poultice for its throat, but I didn’t get a glimpse of it.”

“For God’s sake, Doctor, what is it?”

“I can’t tell you yet, Major. So far I can tell, it is something new to science and I am not sure just what it looks like. However, I hope to be able to show it to you shortly. Is there a telegraph office here?”

“No, but we have a Signal Corps detachment with us, and they have a portable radio set which will put us in touch with the army net.”

“Good! Can you place a tent at my disposal?”

“Certainly, Doctor.”

“All right, I’ll go there, and I would appreciate it if you would send the radio operator to me. I want to send a message to the Bureau of Standards to forward me some apparatus which I need.”

“I’ll attend to it, Doctor. Have you any special advice to give me about the guarding?”

“Yes. Have you, or can you get, any live stock?”

“Live stock?”

“Yes. Cattle preferred, although hogs or sheep will do at a pinch. Sheep will do quite well.”

“I’ll see what I can do, Doctor.”

“Get them by all means, if it is possible to do so. Don’t worry about paying for them: secret service funds are not subject to the same audit that army funds get. If you can locate them, drive a couple of cattle or half a dozen sheep well into the cave and tether them there. If you don’t get them, have your sentries posted well away from the cave mouth, and if any disturbance occurs during the night, tell them to break and run. I hope it won’t come out, but I can’t tell.”

A herd of cattle was soon located and two of the beasts driven into the cave. Two hours later a series of horrible screams and bellowings were heard in the cave. Following their orders the sentries abandoned their posts and scattered, but the noise came no nearer the mouth, and in a few minutes silence again reigned.

“I hope that will be all that will be needed for a couple of days,” said the doctor to the commanding officer, “but you had better have a couple more cattle driven in in the morning. We want to keep the brute well fed. Is there a tank stationed at Fort Thomas?”

“No, there isn’t.”

“Then radio Washington that I want the fastest three-man tank that the army has sent here at once. Don’t bother with military channels, radio direct to the Adjutant General, quoting the Secretary of the Treasury as authority. Tell him that it’s a rush matter, and sign the message ‘Bird’ if you are afraid of getting your tail twisted.”

Twice more before the apparatus which the doctor had ordered from Washington arrived cattle were driven into the depths of the cave, and twice were the screams and bellowings from the cave repeated. Each time searching parties found the cattle gone in the morning. A week after the doctor’s arrival, a special train came up, carrying four mechanics from the Bureau of Standards, together with a dozen huge packing cases. Under the direction of the doctor the cases were unpacked and the apparatus put together. Before the assembly had been completed the tank which had been requested arrived from Camp Meade, and the Bureau mechanics began to install some of the assembled units in it.