"Are you serious?" asked Bigman.

"Quite serious," said David.

"You mean"-he tried to smile-"there really are Martians?"

"Would you believe me if I said there were?"

"No. He came to a sudden decision. "But that doesn't matter. I said I wanted to be in this, and I don't back out." Once again the car moved forward.

The feeble dawn of the Martian heavens was beginning to light the grim landscape when the car approached the fissure. It had been creeping for half an hour previous, its powerful headlights probing the darkness, lest, as Bigman had put it, they find the fissure a little too quickly.

David climbed out of the car and approached the giant crack. No light penetrated it as yet. It was a black and ominous hole in the ground, stretching out of sight in either direction, with the opposing lip a featureless gray prominence. He pointed his flash downward and the beam of light faded into nothing.

Bigman came up behind him. "Are you sure this is the right place?"

David looked about him. "According to the maps, this is the closest approach to a cavern. How far are we from the nearest farm section?"

"Two miles easy."

The Earthman nodded. Farmboys were unlikely to touch this spot except possibly during checkup.

He said, "No use waiting then."

Bigman said, "How are you going to do it, anyway?"

David had already lifted the box which Bigman had obtained in Wingrad City out of the car. He tore it open and took out the contents. "Ever see one of these?" he asked.

Bigman shook his head. He twiddled a piece of It between gloved thumb and forefinger. It consisted of a pair of long ropes with a silky sheen connected at twelve-inch intervals by crosspieces.

"It's a rope ladder, I suppose," he said.

"Yes," said David, "but not rope. This is spun silicone, lighter than magnesium, stronger than steel, and barely affected by any temperatures we're likely to meet on Mars. Mostly, it's used on the Moon, where the gravity is really, low and the mountains really high. On Mars, there's not much use for it because it's a rather flat world. In fact, it was a stroke of luck that the Council could locate one in the city."

"What good will this do you?" Bigman was running the length of it through his hands until the ladder ended in a thick bulb of metal.

"Careful," said David. "If the safety catch isn't on, you can damage yourself pretty badly.".

He took it gently out of Bigman's hand, encircled the metal bulb with his own strong hands, and twisted each hand In opposing directions. There was a sharp little click, but when he released his hold, the bulb seemed unchanged.

"Now look." The soil of Mars thinned and vanished at the approaches of the fissure, and the cliff edge was naked rock. David bent and, with a light pressure, touched the bulb end of the ladder to the crag, faintly ruddy in the flushing sky of morning. He took his hand away, and it remained there, balanced at an odd angle.

"Lift it up," he said..

Bigman looked at him, bent, and lifted. For a moment he looked puzzled as the bulb remained where it was; then he yanked with all his might and still nothing happened.

He looked up angrily. "What did you do?"

David smiled. "When the safety is released, any pressure at the tip of the bulb releases a thin force-field about twelve inches long that cuts right into the rock. The end of the field then expands outward in each direction about six inches, to make a 'T' of force. The limits of the field are blunt, not sharp, so you can't loosen it by yanking it from side to side. The only way you can pull out the bulb is to break the rock clean off."

"How do you release it?"

David ran the hundred-foot length of ladder through his hands and came up with a similar bulb at the other end. He twisted it, then pushed it at the rock. It remained there, and after some fifteen seconds the first bulb fell on its side.

"If you activate one bulb," he said, "the other is automatically deactivated. Or, of course, if you adjust the safety catch of an activated bulb"-he bent down and did so-"it is deactivated"-he lifted it up-"and the other remains unaffected."

Bigman squatted. Where the two bulbs had been there were now narrow cuts about four inches long in the living rock. They were too narrow for him to insert his fingernail.

David Star was speaking. He said, "I've got water and food for a week. I'm afraid my oxygen won't last more than two days, but you wait a week anyway. If I'm not back then, this is the letter you're to deliver to the Council headquarters."

"Hold on. You don't really think these fairy-tale

Martians.... "

"I mean lots of things. I mean I may slip. The rope ladder may be faulty. I may accidentally anchor it to a point at which there is a fault in the rock. Anything. So can I rely on you?"

Bigman looked disappointed. "But that's a fine situation. Am I supposed to sit around up here while you take all the risks?"

"It's the way a team works, Bigman. You know that."

He was stooping at the lip of the fissure. The sun was edging over the horizon before them and the sky had faded from black to purple. The fissure, however, remained a forbidding dusky abyss. The sparse atmosphere of Mars did not scatter light very well, and only when the sun was directly overhead was the eternal night of the fissure dispelled.

Stolidly David tossed the ladder into the fissure. Its fiber made no noise as it swung against the rock, upheld by the knob which held tightly to the stony lip. A hundred feet below they could hear the other knob thump once or twice.

David yanked at the rope to test its hold, then, seizing the topmost rung with his hands, he vaulted into the abyss himself. It was a feathery feeling floating down at less than half the speed one would have on Earth, but there it ended. His actual weight was not far below Earth normal, considering the two oxygen cylinders he carried, each the largest size available at the farm.

His head projected above surface. Bigman was staring at him, wide-eyed. David said, "Now get away and take the car with you. Return the films and viewers to the Council and leave the scooter."

"Right," said Bigman. All cars carried emergency four-wheeled platforms that could travel fifty miles under their own power. They were uncomfortable and no protection at all against cold or, worse still, against dust storms. Still, when a sand-car broke down miles from home, scooters were better than waiting to be found.

David Starr looked downward. It was too dark to see the end of the ladder, the sheen of which glimmered into grayness. Allowing his legs to dangle free, he scrambled down the face of the cliff rung by rung, counting as he did so. At the eightieth rung he reached for the free end of the ladder and reeled it in after hooking an arm about and through a rung, leaving both hands free.

When the lower bulb was in his hand, he reached to the right and thrust it at the face of the cliff. It remained there. He yanked hard at it, and it held. Quickly he swung himself from his previous position to the branch of the rope ladder now dangling from the new anchor. One hand remained on the portion of the ladder he had left, waiting for it to give. When it did so, he swung it outward, so that the bulb from above would swing wide of himself as it fell.

He felt a slight pendulum effect upon himself as the bulb, which had been at the lip of the fissure thirty seconds before, now lashed back and forth some one hundred and eighty feet below the surface of Mars. He looked up. There was a broad swath of purple sky to be seen, but he knew it would get narrower with each rung he descended.

Down he went, and at every eighty rungs he set himself a new anchor, first to the right of the old one and then to the left, maintaining in general a straight passage downward.

Six hours had passed, and once again David paused for a bite of concentrated ration and a swig of water from his canteen. Catching his feet in rungs and relaxing the pressure on his arms was all he could do in the way of resting. Nowhere in all the descent had there been a horizontal ledge large enough for him to catch his breath upon. At least nowhere within the reach of Ms flashlight.

That was bad in other ways. It meant that the trip upward, supposing that there ever was to be a trip upward, would have to be made by the slow method of jabbing each bulb, in turn, at a spot as high as one could reach. It could be done and had been-on the Moon. On Mars the gravity was more than twice what it was on the Moon, and progress would be horribly slow, far slower than the journey down was. And that, David realized grimly, was slow enough. He could not be much more than a mile below surface.

Downward there was only black. Above, the now narrow streak of sky had brightened. David decided to wait. It was past eleven by his Earth-time watch, and that had fair significance on Mars, where the period of rotation was only half an hour longer than on Earth. The sun would soon be overhead.

He thought soberly that the maps of the Martian caverns were at best only rough approximations from the action of vibration waves under the planet's surface. With very slight errors existing he could be miles away from the true entrance into the caverns.

And then, too, there might be no entrances at all. The caverns might be purely natural phenomena, like the Carlsbad Caverns on Earth. Except, of course, that these Martian caverns were hundreds of miles across.

He waited, almost drowsily, hanging loosely over nothing, in darkness and silence. He flexed his numbed fingers. Even under the gloves, the Martian cold nipped. When he was descending, the activity kept him warm; when he waited, the cold burrowed in.

He had almost decided to renew his climbing to keep from freezing when he caught the first -approach of dim light. He looked up and saw the slowly descending dim yellow of sunlight. Over the lip of the fissure, into the small streak of sky that remained to his vision, the sun came. It took ten minutes for the light to increase to maximum, when the entire burning globe had become visible. Small though it was to an Earthman's eyes, its width was one quarter that of the fissure opening. David knew the light would last half an hour or less and that the darkness would return for twenty-four hours thereafter.

He looked about rapidly, swinging as he did so. The wall of the fissure was by no means straight. It was jagged, but it was everywhere vertical. It was as though a cut had been made into the Martian soil with a badly crimped knife, but one which cut straight down. The opposite wall was considerably closer than it had been at the surface, but David estimated that there would be at least another mile or two of descent before it would be close enough to touch.

Still, it all amounted to nothing. Nothing!

And then he saw the patch of blackness. David's breath whistled sharply. There was considerable blackness elsewhere. Wherever an outcropping of rock cast a shadow, there was blackness. It was just that this particular patch was rectangular. It had perfect, or what seemed to be perfect, right angles. It had to be artificial. It was like a door of some sort set into the rock.

Quickly he caught up the lower knob of the ladder, set it as far out in the direction of the patch as he could reach, gathered in the other knob as it fell, and set it still farther out in the same direction. He alternated them as rapidly as he could, hoping savagely that the sun would hold out, that the patch itself was not, somehow, an illusion.

The sun had crossed the fissure and now touched the lip of the wall from which he dangled. The rock he faced, which had been yellow-red, turned gray again. But there was still light upon the other wall, and he could see well enough. He was less than a hundred feet away, and each alternation of ladder knobs brought him a yard closer.

Glimmering, the sunlight traveled up the opposite wall, and the dusk was closing in when he reached the edge of the patch. His gloved ringers closed upon the edge of a cavity set into the rock. It was smooth, The line had neither fault nor flaw. It had to be made by intelligence.

He needed sunlight no longer. The small beam of the flashlight would be enough. He swung his ladder into the inset, and when he dropped a knob he felt it clunk sharply on rock beneath. A horizontal ledge!

He descended quickly, and in a few minutes found himself standing on rock. For the first time in more than six hours he was standing on something solid. He found the inactive bulb, thrust it into rock at waist level, brought down the ladder, then adjusted the safety latch and pulled out the bulb. For the first time in more than six hours both ends of the ladder were free.

David looped the ladder around his waist and arm and looked about. The cavity in the face of the cliff was about ten feet high and six across. With his flashlight pointing the way, he walked inward and came face to face with a smooth and quite solid stone slab that barred farther progress.

It, too, was the work of intelligence. It had to be. But it remained an effective barrier to further exploration just the same.

There was a sudden pain in his ears, and he spun sharply. There could be only one explanation. Somehow the air pressure about himself was increasing.

He moved back toward the face of the cliff and was not surprised to find that the opening through which he had come was barred by rock which had not previously been there. It had slid into place without a sound.

His heart beat quickly. He was obviously in an air lock of some sort. Carefully he removed his nose-piece and sampled the new air. It felt good in his lungs, and it was warm.

He advanced to the inner slab of rock and waited confidently for it to lift up and away.

It did exactly that, but a full minute before it did so David felt his arms compressed suddenly against his body as though a steel lasso had been thrown about him and tightened. He had time for one startled cry, and then his legs pushed one against the other under similar pressure.

And so it was that when the inner door opened and the way to enter the cavern was clear before Mm, David Starr could move neither hand nor foot.