brick and surrounded by tanks of heavy water. This is the only place in the Solar System, I imagine, where cosmic rays never come."

"You mean—"

Lang knocked out his pipe and left it in a gobboon. He opened the lockers to reveal a set of airsuits, complete with helmets and oxygen tanks. "We put these on before going any further," he said.

"Infection on the other side?"

"We're the infected ones. Come on, I'll help you."

As they scrambled into the equipment, Lang added conversationally: "This place has to have all its own stuff, of course ... its own electric generators and so on. The ultimate power source is isotopically pure carbon burned in oxygen. We use a nuclear reactor to create the magnetic field itself, but no atomic energy is allowed inside it." He led the way into the airlock, closed it, and started the pumps. "We have to flush out all the normal air and substitute that from the inner chambers."

"How about food? Barwell said food was prepared in the kitchens and brought here."

"Synthesized out of elements recovered from waste products. We do cook it topside, taking precautions. A few radioactive atoms get in, but not enough to matter as long as we're careful. We're so cramped for space down here we have to make some compromises."

"I think—" Radek fell silent. As the lock was evacuated, his unjointed airsuit spreadeagled and held him prisoner, but he hardly noticed. There was too much else to think about, too much to grasp at once.

Not till the cycle was over and they had gone through the lock did he speak again. Then it came harsh and jerky: "I begin to understand. How long has this gone on?"

"It started about 200 years ago ... an early Institute project." Lang's voice was somehow tinny over the helmet phone. "At that time, it wasn't possible to make really pure isotopes in quantity, so there were only limited results, but it was enough to justify further research. This particular set of chambers and chemical elements is 150 years old. A spectacular success, a brilliant confirmation, from the very beginning ... and the Institute has never dared reveal it. Maybe they should have, back then—maybe people could have taken the news—but not now. These days the knowledge would whip men into a murderous rage of frustration; they 

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