David said, "Quiet, man!"

Bigman's voice sank. "I've seen you in video reels often enough. But why don't your wrists show the mark? I've heard all the members of the Council were marked."

"Where did you hear this? And who told you the library at Canal and Phobos is the Council of Science?"

Bigman flushed. "Don't look down at the farmboy, mister. I've lived in the city. I've even had schooling."

"My apologies. I didn't mean it that way. Will you still help me?"

"Not until I understand about your wrists."

"That's not hard. It's a colorless tattoo that will turn dark in air, but only if I want it to."

"How's that?"

"It's a matter of emotion. Each human emotion is accompanied by a particular hormone pattern in the blood. One and only one such pattern activates the tattoo. I happen to know the emotion that fits."

David did nothing visibly, but slowly a patch on the inner surface of his right wrist appeared and darkened. The golden dots of the Big Dipper and Orion glowed momentarily and then the whole faded rapidly.

Bigman's face glowed and his hands came down for that automatic smack against his boots. David caught Ms arms roughly.

"Hey," said Bigman.

"No excitement, please. Are you with me?"

"Sure I'm with you. I'll be back tonight with the stuff you want and I'll tell you where we can meet.

There's a place outside, near the Second Section.... "

He went on, whispering directions.

David nodded. "Good. Here's the envelope."

Bigman took it and inserted it between his hip boot and thigh. He said, "There's a pocket on the inside top of the better-quality hip boots, Mr. Starr. Do you know that?"

"I do. Don't look down at this farmboy, either. And my name, Bigman, is still Williams. That leaves just one last statement. The Council librarians will be the only ones who will be able to open that envelope safely. If anyone else tries, he'll be hurt."

Bigman drew himself up. "No one else will open it. There are people who are bigger than I am. Maybe you think I don't know that, but I do. Just the same, bigger or not, nobody, and I mean nobody, will take this from me without killing me. What's more, I wasn't thinking of opening it myself, either, if you've given that any thought."

"I have," said David. "I try to give all possibilities some, thought, but I didn't give that one very much."

Bigman smiled, made a mock pass with his fist at David's chin, and was gone.

It was almost dinnertime when Benson returned. He looked unhappy and his plump cheeks were drooping.

He said listlessly, "How are you, Williams?"

David was washing his hands by dipping them into the special detergent solution which was universally used on Mars for this purpose. He withdrew his hands into the stream of warm air for drying, while the wash water flushed away into the tanks where it could be purified and returned to the central supply. Water was expensive on Mars and was used and reused wherever possible.

David said, "You look tired, Mr. Benson."

Benson closed the door carefully behind him. He blurted it out. "Six people died yesterday of the poisoning. That's the highest number yet for a single day. It's getting worse all the time and there's nothing we seem to be able to do."

He glowered at the lines of animal cages. "All alive, I suppose."

"All alive," said David.

"Well, what can I do? Every day Makian asks me if I have discovered anything. Does he think I can find discoveries under my pillow in the morning? I was in the grain bins today, Williams. It was an ocean of wheat, thousands and thousands of tons all set for shipment to Earth. I dipped into it a hundred times. Fifty grains here; fifty grains there. I tried every corner of every bin. I had them dip twenty feet down for samples. But what good is it? Under present conditions it would be a generous estimate to suppose that one out of a billion grains is infected."

He nudged at the suitcase he had brought with him. "Do you think the fifty thousand grains I've got here have the one in a billion among them? One chance in twenty thousand!"

David said, "Mr. Benson, you told me that no one ever died on the farm here, even though we eat Martian food almost exclusively."

"Not as far as I know."

"How about Mars as a whole?"

Benson frowned. "I don't know. I suppose not or I would have heard of it. Of course life isn't as tightly organized here on Mars as it is on Earth. A farmboy dies and usually he is simply buried without formality. There are few questions." Then, sharply, "Why do you ask?"

"I was just thinking that if it were a Martian germ, people on Mars might be more accustomed to it than Earth people. They might be immune."

"Well! Not a bad thought for a non-scientist. In fact, it's a good idea. I'll keep it in mind," He reached up to pat David's shoulder. "You go on and eat. We'll begin feeding the new samples tomorrow."

As David left, Benson turned to his suitcase and was lifting out the carefully labeled little packets, one of which might hold the all-important poisoned kernel. By tomorrow those samples would be ground, each little pile of powder carefully mixed and painstakingly divided into twenty sub-samples, some for feeding and some for testing.

By tomorrow! David smiled tightly to himself. He wondered where he would be tomorrow. He even wondered if he would be alive tomorrow.

The farm dome lay asleep like a giant prehistoric monster curled upon the surface of Mars. The residual fluorescents were pale glimmers against the dome roof. Amid the silence the ordinarily unheard vibrations of the dome's atmospherics, which compressed Martian atmosphere to the normal Earth level and added moisture and oxygen from the quantities supplied by the growing plants of the vast greenhouses, sounded in a low grumble.

David was moving quickly from shadow to shadow with a caution that was, to a large extent, not necessary. There was no one watching. The hard composition of the dome was low overhead, bending rapidly to the ground, when he reached Lock 17. His hair brushed it.

The inner door was open and he stepped inside. His pencil flashlight swept the walls within and found the controls. They weren't labeled, but Bigman's directions had been clear enough. He depressed the yellow button. There was a faint click, a pause, and then the soughing of air. It was much louder than it had been on the day of the checkup, and since the lock was a small one designed for three or four men rather than a giant one designed for nine sand-cars, the air pressure dropped much more quickly.

He adjusted his nosepiece, waited for the hissing to die away, the silence indicating pressure equilibrium. Only then did he depress the red button. The outer section lifted and he stepped out.

This time he was not trying to control a car. He lowered himself to the hard, cold sands and waited for the stomach-turning sensations to pass as he accustomed himself to the gravity change. It took scarcely two minutes for that to happen. A few more gravity-change passages, David thought grimly, and he would have what the farmboys called "gravity legs."

He rose, turned to get his bearings, and then found himself, quite involuntarily, frozen in fascination!

It was the first time he had ever seen the Martian night sky. The stars themselves were the old familiar ones of Earth, arranged in all the familiar constellations. The distance from Mars to Earth, great though it was, was insufficient to alter perceptibly the relative positions of the distant stars. But though the stars were unchanged in position, how vastly they were changed in brilliance.

The thinner air of Mars scarcely softened them, but left them hard and gem-bright. There was no moon, of course, not one such as Earth knew. Mars's two satellites, Phobos and Deimos, were tiny things only five or ten miles across, simply mountains flying loose in space. Even though they were much closer to Mars than the Moon was to Earth, they would show no disk and be only two more stars.

He searched for them, even though he realized they might easily both be on the other side of Mars. Low on the western horizon he caught something else. Slowly he turned to it. It was by far the brightest object in the sky, with a faint blue-green tinge to it that was matched for beauty by nothing else in the heavens he watched. Separated from it by about the width of Mars's shrunken sun was another object, yellower, bright in itself but dwarfed by the much greater brillance of its neighbor.

David needed no star map to identify the double object. They were the Earth and the Moon, the double "evening star" of Mars.

He tore his eyes away, turned toward the low outcropping of rock visible in the light of his pencil flash, and began walking. Bigman had told him to use those rocks as a guide. It was cold in the Martian night, and David was regretfully aware of the heating powers of even the Martian sun, one hundred and thirty million miles away.

The sand-car was invisible, or nearly so, in the weak starlight, and he heard the low, even purr of its engines long before he saw it.

He called, "Bigman!" and the little fellow popped out of it.

"Space!" said Bigman. "I was beginning to think you were lost."

"Why is the engine running?"

"That's easy. How else do I keep from freezing to death? We won't be heard, though. I know this place."

"Do you have the films?"

"Do I? Listen, I don't know what you had in the message you sent but they had five or six scholars circling me like satellites. It was 'Mr. Jones this' and "Mr. Jones that.' I said, 'My name's Bigman,' I said. And then it was 'Mr. Bigman, if you please.' Anyway"-Bigman ticked items off on his fingers-"before the day was gone, they had four films for me, two viewers, a box as big as myself which I haven't opened, and the loan (or maybe the gift for all I know) of a sand-car to carry it all in."

David smiled but made no answer. He entered Into the welcome warmth of the car and quickly, in a race to outrun the fleet night, adjusted the viewers for projection and inserted a film in each. Direct viewing would have been quicker, more preferable, but even in the warm interior of the sand-car his nosepiece was still a necessity, and the bulbous transparent covering of his eyes made direct viewing impossible.

Slowly the sand-car lurched forward through the night, repeating almost exactly the route of Griswold's subsection on the day of the checkup.

"I don't get it," said Bigman. He had been muttering under his breath uselessly for fifteen minutes and now he had to repeat his louder statement twice before the brooding David would respond.

"Don't get what?"

"What you're doing. Where you're going. I figure this is my business because I'm going to stay with you from here on. I've been thinking today, Mr. St- Williams, thinking a lot. Mr. Makian's been in a kind of biting temper for months now, and he wasn't a bad joe at all before that. Hermes came in at that time, with a new shuffle for all hands. And Schoolboy Benson gets his licks in all of a sudden. Before it all started he was nobody, and now he's real pally with the big shots. Then, to top it off, you're here, with the Council of Science ready to put up anything you want. It's something big, I know, and I want to be in on it"

"Do you?" said David. "Did you see the maps I was viewing?"

"Sure. Just old maps of Mars. I've seen them a million times."

"How about the ones with the crosshatched areas? Do you know what those areas stood for?"

"Any farmboy can tell you that. There are supposed to be caverns underneath, except that I don't believe it. My point is this. How in Space can anyone tell there are holes two miles underneath the ground if no one's been down there to see? Tell me that."

David did not bother to describe the science of seismography to Bigman. Instead, he said, "Ever hear of Martians?" Bigman began, "Sure. What kind of a question.... " and then the sand-car screeched and trembled as the little fellow's hands moved convulsively on the wheels. "You mean real Martians? Mars Martians; not people Martians Eke us? Martians living here before people came?"

His thin laugh rattled piercingly inside the car and when he caught his breath again (it is difficult to laugh and breathe at the same time with a nosepiece on), he said, "You've been talking to that guy Benson."

David remained gravely unruffled at the other's glee. "Why do you say that, Bigman?"

"We once caught him reading some kind of book about it, and we ribbed the pants off him. Jumping Asteroids, he got sore. He called us all ignorant peasants, and I looked up the word in the dictionary and told the boys what it meant. There was talk of mayhem for a while, and he got shoved around sort of by accident, if you know what I mean, for a while after that. He never mentioned anything about Martians to ms after that; wouldn't have had the nerve. I guess, though, he figured you were an Earthman and would fall for that kind of comet gas."

"Are you sure it's comet gas?"

"Sure. What else can it be? People have been on Mars for hundreds and hundreds of years. No one's ever seen Martians."

"Suppose they're down in the caverns two miles underneath."

"No one's seen the caverns either. Besides, how would the Martians get there in the first place? People have been over every inch of Mars and there sure aren't staircases going down anywhere. Or elevators, either."

"Are you certain? I saw one the other day."

"What?" Bigman looked back over his shoulder. He said, "Kidding me?"

"It wasn't a staircase, but it was a hole. And it was at least two miles deep."

"Oh, you mean the fissure. Nuts, that doesn't mean anything. Mars if full of fissures."

"Exactly, Bigman. And I've got detailed maps of the fissures on Mars too. Right here. There's a funny thing about them which, as nearly as I can tell from the geography you brought me, hasn't been noticed before. Not a single fissure crosses a single cavern."

"What does that prove?"

"It makes sense. If you were building airtight caverns, would you want a hole in the roof? And there's another coincidence. Each fissure cuts close to a cavern, but without ever touching, as though the Martians used them as points of entrance into the caverns they were building."

The sand-car stopped suddenly. In the dim light of the viewers, which were still focused on two maps projected simultaneously upon the flat white surface of the built-in screens, Bigman's face blinked somberly at David in the back seat.

He said, "Wait a minute. Wait a jumping minute. Where are we going?"

"To the fissure, Bigman, About two miles past the place where Griswold went over. That's where it gets nearest the cavern under the Makian farms."

"And once we get there?"

David said calmly, "Once we get there, why, I'll climb down into it."