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.

David waited. There was no use in speaking to empty air. Presumably the entities who had built the caverns and who could so immobilize him in so immaterial a fashion would be perfectly capable of playing all the cards.

He felt himself lift from the ground and slowly tip backward until the line of his body was parallel with the floor. He tried to crane his head upward but found it to be nearly immovable. The bonds were not so strong as those which had tightened about his limbs. It was rather like a harness of velvety rubber that gave, but only so far.

He moved inward smoothly. It was like entering warm, fragrant, breathable water. As his head left the air lock, the last portion of his body to do so, a dreamless sleep closed over him.

David Starr opened his eyes with no sensation of any passage of time but, with the sensation of life near by. Exactly what form that sensation took he could not say. He was first conscious of the heat. It was that of a hot summer day on Earth. Second, there was the dim red light that surrounded him and that scarcely sufficed for vision. By it he could barely make out the walls of a small room as he turned his head. Nowhere was there motion; nowhere life.

And yet somewhere near there must be the working of a powerful intelligence. David felt that in a way he could not explain.