There was a hidden susurrus of machinery as they stepped out into a small bare entryroom. Another guard sat there, beside a desk. Past him was a small door of immense solidity, the door of a vault. "You'll have to leave everything metallic here," said Lang. "A steel object could jump so fiercely as to injure you. Your watch would be ruined. Even coins could get uncomfortably hot ... eddy currents, you know. We're about to go through the strongest magnetic field ever generated." Silently, dry-mouthed, Radek piled his things on the desk. Lang operated a combination lock on the door. "There are nervous effects too," he said. "The field is actually strong enough to influence the electric discharges of your synapses. Be prepared for a few nasty seconds. Follow me and walk fast." The door opened on a low, narrow corridor several meters long. Radek felt his heart bump crazily, his vision blurred, there was panic screaming in his brain and a sweating tingle in his skin. Stumbling through nightmare, he made it to the end. The horror faded. They were in another room, with storage facilities and what resembled a spaceship's airlock in the opposite wall. Lang grinned shakily. "No fun, is it?" "What's it for?" gasped Radek. "To keep charged particles out of here. And the whole set of chambers is 500 meters underground, sheathed in ten meters of lead 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.