It came out of the metal wall. The skit-trees were in the wall. They came out of it. He saw another that went into it.

He went back to the flier and climbed in. He turned the communicator up to maximum power. The racket that came out of it was deafening. He punched the call-button. Again and again and again. Nothing happened. He turned the set off. The dead stillness which followed was daunting.

"Well?" said Haynes.

"It's impossible," said Wentworth, "but I can explain everything. That wall isn't real."

"Then we ram through it?"

"We'd kill ourselves!" Wentworth told him, exasperated. "It's solid!"

"Not real, but solid?" asked Haynes. "A bit unusual, that. When I get back to Earth and am a happily married man, I'll try to have a more plausible story than that to tell my wife if I ever come home late, not that I ever will."

Wentworth looked at him. And Haynes grinned. But there was sweat on his face. Wentworth grunted.

"I'm scared too, but I don't make bad jokes to cover up," he said sourly. "This can be licked. It's got to be!"

"What is it?"

"How do I know?" demanded Wentworth. "It makes sense, though. A city that vanishes and re-appears, apparently without anybody in it. That doesn't happen. This can—this tank we're in. There wasn't any machinery around to put up a wall like this.

"The top wasn't heaved into place either. It wasn't lowered down to seal us in. It didn't slide into position. One instant it wasn't there, and the next instant it was. Like something that—hm—had materialized out of nowhere. Maybe that's it! And the city was the same sort of trick! Maybe that's the secret of this whole civilization we're trying to trace!"

His voice echoed weirdly against the metal ceiling on every hand.

"What's the secret?"

"Materializing things! Making a—synthetic sort of matter! Making—well—force-fields that look and act like substance! Of course! If you can generate a building, why build one? We can make a magnetic field with a coil of wire and an electric current. It's just as real as a brick. It's simply different from a brick.

"We can make a picture 

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