There was a tiny shock; in a minute, trivial contact of the ship with something outside it. Drifting within the now brightly lighted bore, it had touched the wall. There was no force to the impact.

Keller made an interested noise. When eyes turned to him, he pointed to a dial. A needle on that dial pointed just past the figure "30." Burke grunted.

"The devil! We've been waiting for things to happen, and they already have! It's our move."

"According to that needle," agreed Holmes, "somebody has kindly put thirty point seven mercury inches of air-pressure around the ship outside. We can walk out and breathe, now."

"If," said Burke, "it's air. It could be something else. I'll have to check it."

He got out the self-contained diving apparatus that had been brought along to serve as a strictly temporary space suit.

"I'll try a cigarette-lighter. Maybe it will burn naturally. Maybe it will go out. It could make an explosion. But I doubt that very much."

"We'll hope," said Holmes, "that the lighter burns."

Burke climbed into the diving suit, which had been designed for amateurs of undersea fishing to use in chilly waters. On Earth it would have been intolerably heavy, for a man moving about out of the ocean. But there was no weight here. If M-387 had a gravitational field at all, which in theory it had to have, it would be on the order of millionths of the pull of Earth.

Keller sat in the control-chair, watching the instruments and the outside television screens which showed the bore now reduced to fifty feet. Somehow the more distant parts of the tunnel looked hazy, as if there were a slight mist in whatever gas had been released in it. Sandy watched Burke pull on the helmet and close the face-plate. She grasped a hand-hold, her knuckles turning white. Pam nestled comfortably in a corner of the ceiling of the control-room. Holmes frowned as Burke went into the air-lock and closed the inner door.

His voice came immediately out of a speaker at the control-desk.

"I'm breathing canned air from the suit," he said curtly.

There were scrapings. The outer lock-door made noises. There was what seemed to be a horribly long wait. Then they heard Burke's voice again.

"I've tried it," he reported. "The lighter burns when it's next to the slightly opened door. I'm opening wide now."

More noises from the air-lock.

"It still burns. Repeat. The lighter burns all right. The tunnel is filled with air. I'm going to crack my face-plate and see how it smells."

Silence, while Sandy went white. But a moment later Burke said crisply, "It smells all right. It's lifeless and stuffy, but there's nothing in it with an odor. Hold on—I hear something!"

A long minute, while the little ship floated eerily almost in contact with the walls about it. It turned slowly. Then there came brisk, brief fluting noises. They were familiar in kind. But this was a short message, of some fifteen or twenty seconds length, no more. It ended, was repeated, ended, was repeated, and went on with an effect of mechanical and parrot-like repetition.

"It's good air," reported Burke. "I'm breathing normally. But it might have been stored for ages. It's stale. Do you hear what I do?"

"Yes," said Sandy in a whisper to the control-room. "It's a call. It's telling us to do something. Come back inside, Joe!"

They heard the outer air-lock door closing and its locking-dogs engaging. The fluting noises ceased to be audible. The inner door swung wide. Burke came into the control-room, his helmet face-plate open. He wriggled out of the diving suit.

"Something picked up the fact that we'd entered. It closed a door behind us. Then it turned on lights for us. Then it let air into the entrance-lock. Now it's telling us to do something."

The ship surged, ever so gently. Keller had turned on an infinitesimal trace of drive. The walls of the bore floated past on the television screens. There was mist in the air outside. It seemed to clear as the ship moved.

Keller made a gratified small sound. They could see the end of the tunnel. There was a platform there. Stairs went to it from the side of the bore. There was a door with rounded corners in the end wall. That wall was metal.

Keller carefully turned the ship until the stairway was in proper position for a landing, if there had been gravitation to make the stairs usable. Very, very gently, he lowered the ship upon the platform.

There was a singular tugging sensation which ceased, came again, ceased, and gradually built up to a perfectly normal feeling of weight. They stood upon the floor of the control-room with every physical sensation they'd felt during one-gravity acceleration on the way out here, and which they'd have felt if the ship were aground on Earth.

"Artificial gravity! Whoever made this knew something!" Burke said.

Pam swallowed and spoke with an apparent attempt at nonchalance.

"Now what do we do?"

"We—look for the people," said Sandy in a queer tone.

"There's nobody here, Sandy!" Burke said irritably. "Can't you see? There can't be anybody here! They'd have signaled us what to do if there had been! This is machinery working. We do something and it operates. But then it waits for us to do something else. It's like—like a self-service elevator!"

"We didn't come here for an elevator ride," said Sandy.

"I came to find out what's here," said Burke, "and why it's signaling to Earth. Holmes, you stay here with the girls and I'll take a look outside."

"I'd like to mention," said Holmes drily, "that we haven't a weapon on this ship. When they shot rockets at us back on Earth, we didn't have even a pea-shooter to shoot back with. We haven't now. I think the girls are as safe exploring as they are here. And besides, we'll all feel better if we're together."

"I'm going!" said Sandy defiantly.

Burke hesitated, then shrugged. He unlatched the devices which kept both doors to the air-lock from being open at the same time. It was not a completely cautious thing to do, but caution was impractical. The ship was imprisoned. It was incapable of defense. There was simply nothing sensible about precautions that couldn't prevent anything.

Burke threw open the outer lock door. One by one, the five of them climbed down to the platform so plainly designed for a ship of space—a small one—to land upon. Nothing happened. Their surroundings were completely uninformative. This landing-platform might have been built by any race on Earth or anywhere else, provided only that it used stairs.

"Here goes," said Burke.

He went to the door with rounded corners. There was something like a handle at one side, about waist-high. He put his hand to it, tugged and twisted, and the door gave. It was not rusty, but it badly needed lubrication. Burke pulled it wide and stared unbelievingly beyond.

Before him there stretched a corridor which was not less than twenty feet high and just as wide. The long, glowing tubes of light that illuminated the ship-tunnel were here, too, fixed in the ceiling. The corridor reached away, straight and unbroken, until its end seemed a mere point in the distance. It looked about a full mile long. There were doorways in both its side walls, and they dwindled in the distance with a monotonous regularity until they, too, were mere vertical specks. One could not speak of the length of this corridor in feet or yards. It was a mile.

It was incredible. It was overwhelming. And it was empty. It shone in the glare of the light tubes which made a river of brilliance overhead. It seemed preposterous that so vast a construction should have no living thing in it. But it was absolutely vacant.

They stared down its length for long seconds. Then Burke seemed to shake himself.

"Here's the parlor. Let's walk in, even if there's no welcoming committee."

His voice echoed. It rolled and reverberated and then diminished very slowly to nothing.

Burke strode forward with Sandy close to him. Pam stared blankly, and instinctively moved up to Holmes. Once they were through the door, the sensation was not that of adventure in a remote part of space, but of being in some strange and impossible monument on Earth. The feeling of weight, if not completely normal, was so near it as not to be noticed. They could have been in some previously unknown structure made by men, at home.

This corridor, though, was not built. It was excavated. Some process had been used which did not fracture the stone to be removed. The surface of the rock about them was smooth. In places it glittered. The doorways had been cut out, not constructed. They were of a size which made them seem designed for the use of men. The compartments to which they gave admission were similarly matter-of-fact. They were windowless, of course, but their strangeness lay in the fact that they were empty, as if to insist that all this ingenuity and labor had been abandoned thousands of years before. Yet from somewhere in the asteroid a call still went out urgently, filling the solar system with plaintive fluting sounds, begging whoever heard to come and do something which was direly necessary.

A long, long way down the gallery there were two specks. A quarter-mile from the entrance, they saw that one of the rooms contained a pile of metal ingots, neatly stacked and bound in place by still-glistening wire. At half a mile they came upon the things in the gallery itself. One was plainly a table with a single leg, made of metal. It was unrusted, but showed signs of use. The other was an object with a hollow top. In the hollow there were twisted, shriveled shreds of something unguessable.

"If men had built this," said Burke, and again his voice echoed and rolled, "that hollow thing would be a stool with a vanished cushion, and the table would be a desk."

Sandy said thoughtfully, "If men had built this, there'd be signs somewhere marking things. At least there'd be some sort of numbers on these doorways!"

Burke said nothing. They went on.

The gallery branched. A metal door closed off the divergent branch. Burke tugged at an apparent handle. It did not yield. They continued along the straight, open way.

They came to a larger-than-usual opening in the side wall. Inside it there were rows and rows and rows of metal spheres some ten feet in diameter. There must have been hundreds of them. Beside the door there was a tiny shelf, with a tinier box fastened to it. A long way farther, they came to what had appeared to be the end of this corridor. But it did not end. It slanted upward and turned and they found themselves in the same corridor on a different level, headed back in the direction from which they had come. Their footsteps echoed hollowly in the still-enormous emptiness. There were other closed doors. Burke tried some. Holmes tried others. They did not open. Keller moved raptly, gazing at this and that.

Everything was strange, but not strange enough to be frightening. One could have believed this place the work of men, except that this was beyond the ability of men to make. There must be miles of vacant rooms carved out of solid rock. They came upon some hundreds of yards of doorways, and in every room on which they opened, there were metal frames about the walls. Holmes said suddenly, "If men had built this place, those could be bunks."

They came to another place where there was dust, and a group of six huge rooms communicating not only with the corridor but with each other. They found hollow metal things like cook pans. They found a hollow small object which could have been a drinking vessel. It was broken. It was of a size suitable for men.

"If men built this," said Holmes again, "these could be mess-halls. But I agree with Sandy that there should be signs."

Yet another closed door. It resisted their efforts to open it, just like the others. Keller put out his hand and thoughtfully touched the stone beside it. He looked astonished.

"What?" asked Burke. He touched the stone as Keller had. It was bitterly, bitterly cold. "The air's warm and the stone's cold! What's this?"

Keller wetted the tip of his finger and rubbed it on the rocky side wall. Instantly, frost appeared. But the air remained warm.

The gallery turned again, and again rose. The third-level passageway was shorter; barely half a mile in length. Here they passed door after door, all open, with each compartment containing a huge and somehow malevolent shape of metal. And beside each doorway there was a little shelf with a small box fastened to it.

"These," said Holmes, "could be guns, if there were any way for them to shoot anything. Just by the look of them I'd say they were weapons."

Burke said abruptly, "Keller, the stone being freezing cold while the air's warm means that this place has been heated up lately. Heat's been poured into it. Within hours!"

Keller considered. Then he shook his head.

"Not heat. Warmed air."

Burke went scowling onward. He followed, actually, the only route that was open. Other ways were cut off by doors which refused to open. Sandy, beside him, noted the floor. It was stone like the walls and ceiling. But it was worn. There were slight inequalities in it, beginning a foot or so from the walls. Sandy envisioned thousands of feet moving about these resonant corridors for hundreds or thousands of years in order to wear away the solid stone in this fashion. She felt age about her—incredible age reaching back to time past imagining, while the occupants of this hollow world swarmed about its interior. Doing what?

Burke considered other things. There were the ten-foot metal spheres, ranged by hundreds in what might be a magazine below. There were the squat and ugly metal monsters which seemed definitely menacing to somebody or something. There were the metal frameworks like bunks. There was no rust, here, which could be accounted for if Keller happened to be right and warmed air had been released lately in corridors which before—for ten thousand years or more—had contained only the vacuum of space. And there were those rooms which could be mess-halls.

These items were subject matter for thought. But if what they hinted at was true, there must be other specialized compartments elsewhere. There must be storerooms for food for those who managed the guns—if they were guns—and the spheres, and lived in the bunk-rooms and ate in the mess-halls. There'd be storerooms for equipment and supplies of all sorts. And again, if Keller were right about the air, there must be enormous pressure-tanks which had held the asteroid's atmosphere under high pressure for millennia, only to warm it and release it within the hour so that those who came by ship could use it.

An old phrase occurred to Burke. "A mystery wrapped in an enigma." It applied to these discoveries. Plainly the release of air had been done without the command of any living creature. There could be none here! As plainly, the signals from space had been begun without the interposition of life. The transmitter which still senselessly flung its message to Earth was a robot. The operation of the ship-lock, the warming of air, the lighting of the ship-lock and the corridors—all had been accomplished by machinery, obeying orders given to the transmitter first by some unguessable stimulus.

But why? Other mysteries aside, there had plainly been meticulous preparation for the welcoming of a ship from space. No, not welcoming. Acceptance of a ship from space. Somebody had been expected to respond to those plaintive fluting noises which went wailing through the solar system. Who were those waited-for visitors expected to be? What were they expected to do? For that matter, what was the purpose of the asteroid itself? What had it been built for? At some time or another it must have contained thousands of inhabitants. What were they here for? What became of them? And when the asteroid was left—abandoned—what conceivable situation was to trigger the transmitter to send out urgent calls, and then a directional guiding-signal the instant the call was answered? When Burke's ship came, the asteroid accepted it without question and carried out mechanical operations to make it possible for that ship's crew to roam at will through it. What activated this mechanism of so many eons ago?

The five newly-arrived humans, three men and two girls, trudged along the echoing gallery cut out of the asteroid's heart. Murmurous sounds accompanied them. Once they came to a place where a whispering-gallery effect existed. They heard their footsteps repeated loudly as if the asteroid inhabitants were approaching invisibly, but no one came.

"I don't like this!" Pam said uneasily.

Then her own voice mocked her, and she realized what it was, and giggled nervously. That also was repeated, and sounded like something which seemed to sneer at them. It was unpleasant.

They came to the end of the gallery. There was a stair leading upward. There was nowhere else to go, so Burke started up, Sandy close behind him, and Holmes and Pam behind them. Keller brought up the rear. They climbed, and small noises began to be audible.

They were fluting sounds. They grew louder as the party from Earth went up and up. They reached a landing, and here also there was a metal door with rounded corners. Through it and from beyond it came the piping notes that Burke had heard in his dream some hundreds of times and that lately had come to Earth from emptiness. The sounds seemed to pause and to begin again, and once more to pause. It was not possible to tell whether they came from one source, speaking pathetically, or from two sources in conversation.

Sandy went utterly white and her eyes fixed upon Burke. He was nearly as pale, himself. He stopped. Here and now there was no trace of ribbony-leaved trees or the smell of green things, but only air which was stuffy and lifeless as if it had been confined for centuries. And there was no sunset sky with two moons in it, but only carved and seamless stone. Yet there were the familiar fluting sounds....

Burke put his hand to the curiously-shaped handle of the door. It yielded. The door opened inward. Burke went in, his throat absurdly dry. Sandy followed him.

And again there was disappointment. Because there was no living creature here. The room was perhaps thirty feet long and as wide. There were many vision-screens in it, and some of them showed the stars outside with a precision of detail no earthly television could provide. The sun glowed as a small disk a third of its proper diameter. It was dimmer, too. The Milky Way showed clearly. And there were very many screens which showed utterly clear views of the surface of the asteroid, all broken, chaotic, riven rock and massy, unoxydized metal.

But there was no life. There were not even symbols of life. There were only machines. They noticed a large transparent disk some ten feet across. Specks of light glowed within its substance. Off at one side an angular metal arm held a small object very close to the disk's surface, a third of the way from its edge. It did not touch the disk, but under it and in the disk there was a little group of bright-red specks which quivered and wavered. They were placed in a strict mathematical arrangement which very, very slowly changed so that it would be hours before it had completed a rotation and had exactly the same appearance again.

The flutings came from a tall metal cone on the floor. Another machine nearby held a round plate out toward the cone. "There's nobody here," said Sandy in a strange voice. "What'll we do now, Joe?"

"This must be the transmitter," he murmured. "The sound-record for the broadcasts must be in here, somehow. It's possible that this plate is a sort of microphone—"

Keller, beaming, pointed to a round spot which quivered with an eerie luminescence. It glowed more brightly and dimmed according to the flutings. Burke said "The devil!" and the round spot flickered up very brightly for an instant.

"Yes," said Burke. "It's a mike. It's quite likely—" the round spot flared up and dimmed with the modulations of his voice—"it's quite likely that what I say goes into the broadcast to Earth."

The cone ceased to emit fluting noises. Burke said very steadily—and the spot flickered violently with the sounds—"I think I am transmitting to Earth. If so, this is Joe Burke. I announce the arrival of my ship at Asteroid M-387. The asteroid has been hollowed out and fitted with an air-lock which admitted our ship. It is a—a—"

He hesitated, and Holmes said curtly, "It's a fortress."

"Yes," said Burke heavily. "It's a fortress. There are weapons we haven't had time to examine. There are barracks for a garrison of thousands. But there is no one here. It has been deserted, but not abandoned, because the transmitter was set up to send out a call when some occasion arose. It seems to have arisen. There is a big plate here which may be a star map, with a scale on which light-years may be represented by inches. I don't know. There are certain bright-red specks on it. They are moving. There is a machine to watch those specks. Apparently it actuated the transmitter to make it call to all the solar system."

Keller suddenly put his finger to his lips. Burke nodded and said curtly, "I'll report further."

Keller flipped over an odd switch with something of a flourish—after which he looked embarrassed. The transmitter went dead.

"He's right," said Holmes. "Back home they know we're here, I suspect, and you've told enough to give them fits. I think we'd better be careful what we say in the clear."

Burke nodded again. "There'll be calls from Earth shortly and we can decide whether or not to use code then. Keller, can you trace the leads to this transmitter and find the receiver that picked up that West Virginia beam-signal and changed the first broadcast to the second? It should be as sensitive as this transmitter is powerful."

Keller nodded confidently.

"It'll take thirty-some minutes for that report of mine to reach Earth and an answer to get back," observed Burke, "if everything works perfectly and the proper side of Earth is turned this way. I think we can be sure there's nobody but us in the fortress."

His sensations were peculiar. It was exciting to have found a fortress in space, of course. It was the sort of thing that might have satisfied a really dedicated scientist completely. Burke realized the importance of the discovery, but it was an impersonal accomplishment. It did not mean, to Burke, that he'd carried out the purpose behind his coming here. This fortress was linked to a dream about a world with two moons in its sky and someone or something running breathlessly behind unearthly swaying foliage. But this place was not the place of that dream, nor did it fulfill it. Mystery remained, and frustration, and Burke was left in the state of mind of a savage who has found a treasure which means much to civilized men, but doesn't make him any happier because he doesn't want what civilized men can give him.

He grimaced and spoke without elation.

"Let's go back to the ship and get a code message ready for Earth."

He led the way out of this room of many motionless but operating machines. The incredibly perfect vision-screen images still portrayed the cosmos outside with all the stars and the sun itself moving slowly across their plates. They saw sunshine and starlight shining on the broken, chaotic outer surface of the asteroid. Wavering, curiously writhing red specks on the ten-foot disk continued their crawling motion. Keller fairly glowed with enthusiasm as he began to investigate this apparatus.

They all went back to the ship, except for Keller. They retraced their way along the long and brilliantly lighted galleries. They descended ramps and went along more brilliantly lighted corridors. Then they came to the branch which had been blocked off by a door that would not open. It was open now. They could see along the new section for a long, long way. They passed places where other doors had been closed, but now were open. What they could see inside them was almost exclusively a repetition of what they saw outside of them. They passed the place where hundreds of ten-foot metal spheres waited for an unknown use. They passed the table with a single leg, and the compartment with many metal ingots stored in it.

Finally, they came to the door with rounded corners, went through it, and there was their ship with its air-lock doors open, waiting in the brightly lighted tunnel.

They went in, and the feeling was of complete anticlimax. They knew, of course, that they had made a discovery beside which all archÆological discoveries on Earth were trivial. They had come upon operating machines which must be old beyond imagining, unrusted because preserved in emptiness, and infinitely superior to anything that men had ever made. They had come upon a mystery to tantalize every brain on Earth. The consequences of their coming to this place would re-make all of Earth's future. But they were singularly unelated.

"I'll make up a sort of report," said Burke heavily, "of what we saw as we arrived, and our landing, and that sort of thing. We'll get it in code and ready for transmission. We can use the asteroid's transmitter."

Holmes scowled at the floor of the little ship.

"You'll make a report, too," said Burke. "You realized that this is a fortress. There can't be any doubt. It was built and put here to fight something. It wasn't built for fun. But I wonder who it was meant to do battle with, and why it was left by its garrison, and why they set up a transmitter to broadcast when something happened! Maybe it was to call the garrison back if they were ever needed. But thousands of years—You make a report on that!"

Holmes nodded.

"You might add," said Pam, shivering a little, "that it's a terribly creepy place."

"What I don't understand," said Sandy, "is why nothing's labelled. Nothing's marked. Whoever built it must have known how to write, in some fashion. A civilized race has to have written records to stay civilized! But I haven't seen a symbol or a pointer or even a color used to give information."

She got out the papers on which she would code the reports as Burke and Holmes turned them over for transmission. She began to write out, carefully, the elaborate key to the coding. Almost reluctantly, Pam prepared to do the same with Holmes' narrative of what he'd seen.

But if enthusiasm was tempered in the ship, there was no such reserve in the United States. Burke's voice had cut into one of the space broadcasts which arrived every seventy-nine minutes. There had been the usual cryptic, plaintive piping noises, repeating for the thousandth time their meaningless message. Then a human voice said almost inaudibly, "... 'll we do now, Joe?" It was heard over an entire hemisphere, where satellite-tracking stations and radar telescopes listened to and recorded every broadcast from space.

It was a stupendous happening. Then Burke's voice came through the flutings. "This must be the transmitter. The sound-record for the broadcasts must be in here, somehow. It's quite possible that this plate is a sort of microphone...." A few seconds later he was heard to say, "The devil!" And later still he addressed himself directly to his listeners on Earth.

He'd spoken the words eighteen and a fraction minutes before they arrived, though they traveled at the speed of light. Broadcast and ecstatically reported in the United States, they touched off a popular reaction as widespread as that triggered by the beginning of the signals themselves. Broadcasters abandoned all other subject matter. Announcers with lovely diction stated the facts and then expanded them into gibbering nonsense. Man had reached M-387. Man had spoken to Earth across two hundred seventy million miles of emptiness. Man had taken possession of a fortress in space. Man now had an outpost, a stepping-stone toward the stars. Man had achieved.... Man had risen.... Man now took the first step toward his manifest destiny, which was to occupy and possess all the thousands of thousands of planets all the way to the galaxy's rim.

But this was in the United States. Elsewhere, rejoicing was much less, especially after a prominent American politician was reported to have said that America's leadership of Earth was not likely ever to be challenged again. A number of the smaller nations immediately protested in the United Nations. That august body was forced to put upon its agenda a full-scale discussion of U.S. space developments. Middle European nations charged that the purpose of America was to monopolize not only the practical means of traveling to other members of the solar system, but all natural and technical resources obtained by such journeyings. With a singular unanimity, the nations at the edge of the Russian bloc demanded that there should be equality of information on Earth. No nation should hold back scientific information. In fact, there was bitter denunciation of the use of code by the humans now on M-387. It was demanded that they answer in the clear all scientific inquiries made by any government—in the clear so everybody could eavesdrop.

In effect, the United States rejoiced in and boasted of the achievements of some of its citizens who, after escaping attack by American guided missiles, had found a stepping-stone toward the stars. But the rest of the world jealously demanded that the United States reap no benefit from the fact. International tension, in fact, rose to a new high.

And Burke and the others laboriously gathered this bit of information and discovered the lack of that. They found incredible devices whose purpose or workings they could not understand. They found every possible evidence of a civilization beside which that of Earth was intolerably backward. But the civilization had abandoned the asteroid.

By the second day the mass of indigestible information had become alarming. They could marvel, but they could not understand. And not to understand was intolerable. They could comprehend that there was a device with red sparks in it which had made another device send a fluting, plaintive call to all the solar system. Nothing else was understandable. The purpose of the call remained a mystery.

But the communicators hummed with messages from Earth. It seemed that every radar telescope upon the planet had been furnished with a transmitter and that every one bombarded the asteroid with a tight beam carrying arguments, offers, expostulations and threats.

"This ought to be funny," said Burke dourly. "But it isn't. All we know is that we've found a fortress which was built to defend a civilization about which we know nothing except that it isn't in the solar system. We know an alarm went off, to call the fortress' garrison back to duty, but the garrison didn't come. We did. We've some evidence that a fighting fleet or something similar is headed this way and that it intends to smash this fortress and may include Earth. You'd think that that sort of news would calm them down, on Earth!"

The microwave receiver was so jammed with messages that there was no communication at all. None could be understood when all arrived at once. Burke had to send a message to Earth in code, specifying a new and secret wavelength, before it became possible to have a two-way contact with Earth. But the messages continued to come out, every one clamoring for something else of benefit to itself alone.