Katherine was also flailing about, and they wound up throwing their arms around each other. The speed of the tube was such, however, that they did not actually fall to the front of the tube. It was accelerating, and Derec felt his ears pop from the sudden change in altitude. He hadn’t even noticed the pressure change on their startling ascent.

Finally, the tube leveled off again, smoothly, and then gently rose again just enough to decelerate and come to an easy stop. Derec lay where he was for a moment, looking at Katherine. She smiled and looked away as they untangled themselves.

The tube door opened and another foreman looked down at them. “Unusual cargo,” the robot said. “You are unharmed?”

Derec and Katherine laughed as they climbed out, nodding in reassurance. He noticed that she had lost her hard edge somewhere on the breathtaking ride.

“There it is,” said Katherine.

The dome rose up right in front of them, the great bronze surface nearly blinding them in the bright sunlight. The dianite had a very fine, pebbled texture, which saved them from an even worse glare. High above them, the curve of the dome carried the top out of sight.

“I don’t see a door anywhere,” said Derec.

They started walking around the base of the dome, looking allover its nearly smooth, unbroken surface. It was even higher than Derec had guessed from a distance. It had no visible seams or openings of any kind.

When the tunnel stop came into view again, they knew they had walked all the way around the base of the dome. Derec stopped, still looking up for any hint of how to enter. He supposed an opening was possible at the top, but placing it there seemed out of character for Robot City.

Katherine brushed her fingertips along the dianite. “It’s pretty. “

“Yeah.” Derec rapped on the hard surface experimentally. “I suppose we could stand out here and shout, but I doubt anyone inside would hear us.”

Katherine faced the dome and backed away, searching the even curve again.

He had taken just a few steps to follow her when he heard a muted ripping sound behind him. When he looked back, he saw the dianite opening in a jagged line where they had been standing, as though invisible hands were tearing it. As they watched, the blue-skinned form of a humanoid robot stepped out.

Katherine drew herself up. “Take us to Keymo,” she ordered firmly.

“This is a security area. What is your business with Keymo?” The robot asked.

“Identify yourself,” she demanded.

“I am Security 1K. What is your business with Keymo?”

“He must give us the Key to Perihelion.”

Derec stepped beside her, afraid that her direct, rather arrogant approach was going to backfire if they didn’t offer some kind of explanation. “According to the Second Law, you must obey our orders. After you take us to Keymo, we will instruct him to hand over the Key. Let’s go.” He started forward confidently, though it was only a bluff.

Security 1K did not take the bluff. It did not move aside at all. “No.”

Derec stepped back, not wanting to challenge the robot’s physical prowess. He knew that the positronic brains in the robots were reliable, so his earlier assumption seemed to be true: the robots were operating under Second Law instructions, certainly from the mysterious holder of the office in the Compass Tower. That suggested a new argument to him.

“Hold it,” said Derec. “Look. Apparently you have a very strong Second Law imperative that you are operating on, established previously. Okay. But that was a general instruction, I’ll bet. Right?”

“That is right. The need for security in this matter is part of the entire project of this facility.”

“But I’m giving you a specific and important order right now. I believe that should override a general instruction relying on broadly based programming.” Actually, he wasn’t sure he believed that at all, but it was worth a try.

Security 1K hesitated. When the positronic brain of a robot paused long enough for a human to notice, the argument had at least been considered worth an internal debate.

“No,” the robot said, after what was for it a considerable length of time. “The earlier imperative stands.”

Derec sighed, but he wasn’t surprised.

“Our well-being is at stake,” Katherine declared. “We must consult with Keymo. Your prevention of this violates the First Law.”

“How?” Security 1K asked.

“We can’t thrive in a city full of robots. We need other people around us.”

As the robot continued to debate with Katherine, Derec looked at the open edge of the dianite. It seemed oddly familiar, especially in its texture, but he couldn’t figure out why. The substance offered no sign of any frame. It looked quite thin, and seemed to constitute the entire wall.

“You are in no danger,” Security 1K was saying. “This is not a First Law problem.”

She glanced at Derec, who shrugged. The robot was backing into the dome again. A moment later, the two sides of the dianite seemed to straighten and grow together.

Carefully, Derec tapped the former opening, afraid it might be hot. It was not, so he ran his hand over the wall in that area. The surface seemed fully integral with the rest of the wall. He looked at Katherine and raised his eyebrows.

“Katherine, whoever’s behind the creation of this city is some kind of genius. Maybe the robots invented this dianite and maybe they didn’t, but somebody created them. This stuff would be worth a fortune off this planet, just like so many other things here.”

She spun away and started walking quickly along the base of the dome.

Astonished, he watched her for a moment, then went into sputtering rage. “What is wrong with you? You’ve been acting crazy all day-come back here!” He ran after her.

Katherine had stiffened at his shouts, and had then begun walking faster. At the sound of his running footsteps, she broke into a run, also. He slowed to a walk, realizing that if she was truly determined not to talk, catching her wouldn’t help any.

Then he whirled angrily and slammed his fist against the wall. “Hey! Open up in there!” He pounded on the dianite a few more times. Then he stepped back, breathing hard.

A new hole tore open in the wall and Security 1K appeared in the opening. It did not step out this time. “Do you have further business here?”

“Yes! Bring Keymo out here!” It felt good to yell at somebody, and the robot couldn’t just walk away.

“If you do not have new reasons to see him, I request that you stop instructing me to listen to you. Do you have new reasons?”

“Uh-” Derec glanced down the way for Katherine, who had stopped to watch. “Well…”

“Please avoid unnecessary contact with this facility,” said Security 1K. It backed away from the opening, which began to heal again.

Derec watched in frustration as the substance quickly closed. On an impulse, he leaned against a solid portion of the wall and pulled off one of his boots. He stuck it into the small portion of the hole that still remained and kept a careful eye on the dianite as it grew together. Now he remembered why it was familiar-the substance was similar to the material out of which these robots were made, possibly even a cellular material. He had had experience with these robot parts when he had created the robot Alpha. That had occurred long before he had reached Robot City, but after his amnesia had come on him. This dianite did not seem to be alive, exactly, but it certainly had some startling properties.

The dianite grew around the boot-and stopped, much to his relief. He had been afraid it would simply keep growing together even if it had to cut right through the boot. Instead, his boot had been incorporated into the wall as part of it.

He leaned down close and prodded the dianite around his boot with his fingers. He was right-the tearing sound had given away the secret. This stuff was very hard as an integral unit, but once the tear was started it was quite fragile, and even grew limp within a short radius of the tear. He was able to pull a few of the modular cells apart with his fingers now. The tear could be opened again.