"True."

"But it didn't die."

"Perhaps some rapid form of immunity—"

"No. Impossible. There are only two explanations. When the child stopped crying, either the organism was no longer there—had been blown away, cleared from the air—or else the organism—"

"Changed," Stone said. "Mutated."

"Yes. Mutated to a noninfectious form. And perhaps it is still mutating. Now it is no longer directly harmful to man, but it eats rubber gaskets."

"The airplane."

Hall nodded. "National guardsmen could be on the ground, and not be harmed. But the pilot had his aircraft destroyed because the plastic was dissolved before his eyes."

"So Burton is now exposed to a harmless organism. That's why the rat is alive."

"That's why Burton is alive," Hall said. "The rapid breathing isn't necessary. He's only alive because Andromeda changed."

"It may change again," Stone said. "And if most mutations occur at times of multiplication, when the organism is growing most rapidly . . ."

The sirens went off, and the computer flashed a message in red.

GASKET INTEGRITY ZERO.

LEVEL V CONTAMINATED AND SEALED.

Stone turned to Hall. "Quick," he said, "get out of here. There's no substation in this lab. You have to go to the next sector."

For a moment, Hall did not understand. He continued to sit in his seat, and then, when the realization hit him, he scrambled for the door and hurried outside to the corridor. As he did so he heard a hissing sound, and a thump as a massive steel plate slid out from a wall and closed off the corridor.

Stone saw it and swore. "That does it," he said. "We're trapped here. And if that bomb goes off, it'll spread the organism all over the surface. There will be a thousand mutations, each killing in a different way. We'll never be rid of it."

Over the loudspeaker, a flat mechanical voice was saying, "The level is closed. The level is closed. This is an emergency. The level is closed."

There was a moment of silence, and then a scratching sound as a new recording came on, and Miss Gladys Stevens of Omaha, Nebraska, said quietly, "There are now three minutes to atomic self-destruct."

29. Three Minutes

A NEW RISING AND FALLING siren came on, and all the clocks snapped their hands back to 1200 hours, and the second hands began to sweep out the time. The stop-clocks all glowed red, with a green line on the dial to indicate when detonation would occur.

And the mechanical voice repeated calmly, "There are now three minutes to self-destruct."

"Automatic," Stone said quietly. "The system cuts in when the level is contaminated. We can't let it happen."

Hall was holding the key in his hand. "There's no way to get to a substation?"

"Not on this level. Each sector is sealed from every other.

"But there are substations, on the other levels?"

"Yes . . ."

"How do I get up?"

"You can't. All the conventional routes are sealed."

"What about the central core?" The central core communicated with all levels.