"And since then?" Peter asked huskily.
"Since then, we've been burrowing. All the big cities.... It would be an impossible task if we tried to include all the thinly-populated areas, of course, but it doesn't matter. By the time we excavate enough to take care of a quarter of the earth's population, the other three-quarters will be dead, or worse."
"I wonder," Peter said shakily, "if I am strong enough to take it."
Arnold laughed harshly. "You are. You've got to be. You're part of our last hope, you see."
"Our last hope?"
"Yes. You're a scientist."
"I see," said Peter. And for the first time, he thought of the Citadel. No plan leaped full-born into his mind, but, maybe, he thought, there's a chance....
It wasn't very big, the thing that had been his shining dream. It lay there in its rough cradle, a globe of raw dura-steel not more than five hundred meters in diameter, where the Citadel was to have been a thousand. It wouldn't house a hundred scientists, eagerly delving into the hinterland of research. The huge compartments weren't filled with the latest equipment for chemical and physical experiment; instead, there was compressed oxygen there, and concentrated food, enough to last a lifetime.
It was a new world, all by itself; or else it was a tomb. And there was one other change, one that you couldn't see from the outside. The solid meters of lead in its outer skin, the shielding to keep out cosmic rays, were gone.