He touched the arm and hand that had been on the other side and couldn't find anything wrong with them. Whatever kept life from thriving on the other side hadn't killed him yet— if it was a poison, it wasn't immediate, and it certainly wasn't the barrier itself.

He reviewed the rules he had learned for crossing the wall. It had to be bare skin. He had to strike it with some force. And if he wanted his whole body through, he'd have to strike the barrier with his whole body at once.

He stripped off his clothing, folding it neatly and laying it on top of his bow and arrows. Then he piled some rocks on top of them so they wouldn't blow away. Silently he hoped that he would indeed need these clothes again.

For a moment he contemplated leaping face first against the wall, but didn't like the idea—striking it with his fist had felt like hitting a wall, after all, and he didn't relish doing that with his face or his groin. Not that it would feel wonderful to do it back-first either, but it was the lesser of two evils.

He walked a ways along the edge of the barrier until he came to a place where there was a fairly steep hill. He walked to the top and then, after a few deep breaths and a whispered farewell to his family, he ran headlong down the hill. Within moments his running was completely out of control, except that when he neared the wall he planted a foot and flung his body in a wild spin designed to lay him flat against the barrier.

Flat was not what he achieved at all. Instead his buttocks passed through first, and then, as he slowed, his thighs and his body up to the shoulders. His arms and head remained outside the barrier, even as his feet fell through and struck the stony ground on the other side. His heels hurt, but he hardly cared about that, because here he stood, his body inside, his arms and head outside.

I've got to get back outside, he thought, and try it again.

Too late. In the last moments before he stopped moving at all, his shoulders had passed inside. He was stuck again as he had been before, unable to bring his body along to follow his arms. The key difference this time was that his head was outside the barrier, and his chin and ears seemed to be reluctant to follow him inside. Worse, he couldn't even bring his arms all the way inside, because he needed the full weight of his body to pull them through, and with his chin hung up on the barrier he couldn't do it.

This has got to be the stupidest way anyone ever found to die, thought Nafai.

Remember your geometry, he told himself. Remember anatomy. My chin may be at too sharp an angle from my neck for me to pull it through, but at the top of my head there's a smooth, continuous curve. If I can just jut my chin forward and pull my head back… assuming that I don't rip my ears off in the process… but those can flex, can't they?

Slowly, laboriously, he tilted his head back and felt himself pulling through. I can do it, he thought. And then my arms will be easy enough.

His head came through all at once, at the end, his face fully inside the barrier. Only his arms continued to protrude through to the outside.

He meant to pull his arms through at once, after resting for just a moment, but as he rested, panting from the exertion, he realized that his need to breathe only increased, and was growing desperate. He was suffocating somehow, even as he drew great draughts of strange-smelling air into his lungs.

Strange-smelling air, dry and cool, and he wasn't getting any oxygen. Even as the panic of suffocation rose within him, his rational mind realized what he should have known all along: The reason that nothing lived behind the barrier was that there was no oxygen in here. It was a place designed to eliminate all decay—and most decay, the rapidest of it, was linked to the presence of oxygen, or oxygen and hydrogen joined to form water. There could be no life, and therefore not even microbes to eat away at surfaces; no water to condense or freeze or flow; no oxidation of metals. And if the atmosphere also failed to support anaerobic life-forms, there'd be little within the barrier to cause decay except sunlight, cosmic radiation, and atomic decay. The barrier had been set up to preserve everything within it, so it could last for forty million years.

This sudden comprehension of the purpose of the barrier was no comfort. For his rational mind was not particularly in control right now. No sooner had he realized that he could not breathe than his hands, still sticking through the barrier, began clutching for air, trying to pull him back through the barrier. But he was in exactly the same situation he had been in before, on the outside, when only an arm was through the wall. He could push his arms deeper into the barrier, but when his face and chest reached the wall, he could go no farther. His hands could touch the breathable air on the other side, but that was all.

Made savage by fear, he beat his head against the barrier, but there simply wasn't leverage—even with panic driving his muscles—to get force enough to push his face through to the breathable air. He really was going to die. Yet he still struck his head against the barrier, again, again, harder.

Perhaps with the last blow he stunned himself; perhaps he was simply weakening from lack of oxygen, or merely losing his balance. Whatever happened, he fell backward, the resistance of the barrier slowing his fall as his arms slipped inside through the invisible wall.

This is fine, thought Nafai. If I can just get to where the slope goes the other way, I can run down toward the barrier and get through again, only this time face first. Even as he thought of this cheerful plan, he knew it wouldn't work. He had spent too long already trying to get through the barrier right here—he had used up too much oxygen inside his own body, and there was no way he had enough left to climb another hill and make another downward run before he blacked out.

His hands came free and he fell backward onto the stony ground.

He must have struck very hard, for to him it sounded like the loudest, longest thunderclap he had ever heard. And then wind tore across his body, picking him up, rolling him, twisting him.

As he gasped in the wind, he could feel that somehow, miraculously, his breathing was working again. He was getting oxygen. He was also getting bruised as the wind tossed him here and there. On the stones. On the grass.

On the grass.

The wind had died down to a gusty breeze—he opened his eyes. He had been flung every which way, perhaps fifty yards. It took him a while to orient himself. But, lying on grass, he knew he was outside the barrier. Was the wind another defense mechanism, then, hurling intruders through the wall? Certainly his body was scraped and bruised enough to bear that interpretation. He could still see a few dust devils whirling in the distance, far within the dead land.

He got up and walked to the barrier. He reached out for it, but it wasn't there. The barrier was gone.

That was the cause of the wind. Atmospheres that had not mixed in forty million years had suddenly combined again, and the pressure must not have been equal on both sides of the barrier. It was like a balloon popping, and he had been tossed about like a scrap of the balloon's skin.

Why had the barrier disappeared?

Because a human passed through it completely. Because if the barrier had not come down, you would have died.

To Nafai it seemed like the voice of the Oversoul inside his head.

(Yes, I'm here, you know me.)

"I destroyed the barrier?"

(No, I did. As soon as you passed all the way through, the perimeter systems informed me that a human being had penetrated. All at once I became aware of parts of myself that had been hidden from me for forty million years. I could see all the barriers, knew at once all their history and understood their purpose and how to control them. If you had been some exceptionally determined intruder who didn't belong here, I would have told the perimeter systems to let you die; they would have immediately been hidden from me again. That has happened twice before, in all these years. But you were the very one I meant to bring here, and so the purpose of the barrier was finished. I ordered its collapse, bringing oxygen to you and, therefore, to the rest of this place.)