in the opposite direction.

As the two men came nearer and nearer, the strangeness in the air
increased. They felt they were being penetrated through and through with
invisible lances, with tiny prickles of heat. "Radiation?" queried Burl
softly, afraid of the answer. His father trudged grimly on for a
moment, and then put down his pack. He took out a Geiger counter and
activated it.

He shook his head. "No radioactivity," he said. "Whatever this is, it
isn't that."

They reached the wall of the building. Oddly, here they seemed sheltered
from the unusual vibrations. Burl realized that the source was above
them, probably the two mighty discs raised high in the sky.

The Dennings surveyed the building, but found no entrance. It must have
been a quarter of a mile around its walls, but there was no sign of a
door or entry. The wall was of a rocklike substance, but it was not like
any rock or plastic Burl had ever seen.

"We've got to get in," said Burl as they returned to the starting point,
"but how?"

His father smiled. "This way." He opened his pack and took two cans of
blasting powder from it. "I thought these would come in handy. Lucky we
had some left over from the blasting we did last week."

He set both cans at the base of the high wall, wired them together, and
ran the wire as far as it reached. When the two men were a safe distance
away, Mark sparked off the explosive.

There was a thunderous roar: rocks and dirt showered around them, and
bits of black powdery stuff. When the smoke cleared, Burl and his father
leaped to their feet, rifles in hand.

There was a crack in the side of the wall where the explosive had gone
off. And the rip was large enough to get through!

Without a word, they charged across the ground, still smoking from the
concussion, and squeezed through the mysterious walls of the enigmatic
building.

The walls were thin, thin but hard, as befit masters of atomic
engineering. Inside, they found a roomless building--one single chamber
within the frame of the outer walls.

A dim, bluish light emanated from the curving ceiling. On the uncleared
rocky ground which was the floor of the building were a number of huge
machines.

They were spherical glassy inventions, many times the height of a man,
connected by strings of thick metal bars and rows of smaller globes,
none of which was familiar. There was a steady humming noise, and above,
the two giant, metal masts penetrating the ceil