"And we in turn thought you were Martians," said Ulkay, through the nozzle. There was more laughter in both groups. "For Pete's sake!" muttered Lloyd. "For Pete's sake! Look, Ulkay, why don't both our groups get some rest and we'll make our inspection tour of the city tomorrow, the two groups together?" Ulkay, after a babble of discussion with his men, was in agreement with this plan, and they and the Earthmen shared a large room within one of the old abandoned buildings. "Will your air supply not run out?" Ulkay queried. "Not on these," Lloyd explained. "They're not tank masks; they're compression masks. A hydraulic system inside the suit keeps a compressor running in this gadget on our backs, as long as we're moving about. Martian air is thin but non-poisonous." "But if you sleep?" "The air runs low, which makes us fidget, which pumps more air through the compressors," Lloyd explained. Ulkay expressed admiration at the cleverness of Earth scientists, and then joined his men in slumber. The Earthmen, tired and happy, fell soundly asleep. * * * * * It was still dark, the chill purple dark of Mars at night, when Lloyd awakened abruptly. His body was tense and his mind keenly alert. Something was wrong. He felt it, but couldn't place the source of his uneasiness. He sat up and looked about him. Starlight, coming in bright pinpoints through a high arched window, glinted reassuringly off the helmets of his men, lying in pools of deep shadow all about him. He looked for Ulkay and his group, and saw their smaller silhouettes huddled on the stone flooring. Feeling a little better, he lay down once more and tried to fall back to sleep. But there was a gnawing, nagging something in his mind that would not allow sleep to come. "What's bothering me?" he asked himself. "Is it something about Ulkay and his bunch? The only really odd thing about them is that they don't wear any breathing equipment in this thin air, right? And didn't Ulkay explain that the atmosphere on Venus is just as thin? It didn't jibe with Harrison's opinion about atmospheres, but Harrison hasn't actually _been_ to Venus, after all, and the cloudiness still keeps its atmosphere a secret from Earth's spectroscopes, right?" His mind assured him that this was right and he felt a little better, but not much. "So what's eating me? A hunch? Intuition? Or just alien-planet nerves?" he went on. "Why should I wake up in the middle of the night feeling scared? Aren't my men all present and accounted for? Aren't they sleeping quietly, just as they should be?"