When the daylight finally came, they had slept again, this time in their space suits, without bothering to set up the tent. Rising to his feet, Paul Arnold pointed suddenly. "Look! An ancient road!" he shouted. It was true. The highway ran there between the hills. A stone ribbon, covered here and there with drifted sand, which showed that there was no traffic of any sort now. The ruins along it looked a little less battered than those which the two men had previously seen, and there were vast lumps of corroded metal, too. Machinery in a former age. "The road goes our way," Harwich commented. "We'll follow it." Hours later, Paul Arnold offered an opinion. "Part of the mystery of Io is clearing up, Evan," he said. "The ruins around here. They're almost identical in architecture to the ruins of Ganymede and the other Jovian satellites. The evidence looks plain. There must have been a single great civilization once, extending over all the moons of Jupiter." Harwich, thinking of, and hating George Bayley for his diabolical treachery, was only half listening. "Yes?" he questioned. "Yes," the boy answered. "And look at those dry ditches, and the big, rusty pumps! The valley here must have been rich, irrigated farmland, once!" They were going across a huge bridge, now, made of porcelain blocks. It was a magnificent structure, magnificently designed according to intricate principles of engineering. "What I can't understand is why all this country became deserted," Paul offered. "You'd think that people who could build things like this would never die out! They could conquer any difficulty that might come up, it would almost seem. Even if their world got old and worn out. After all, even Earthmen can make almost dead worlds artificially habitable again with airdromes, and with imported atmosphere and water." This was another mystery. But it touched Evan Harwich's thoughts only faintly. Nor did he care very much when later Paul pointed out to him rich deposits of ore--outcroppings along the road. He'd seen them himself, and the tunnel mouths, too, of ancient mine workings. There were many fortunes to be won here, in costly metals, just as on the other Jovian satellites. But how could this be important, now, with death dogging their tracks, and so many other things more important, to be concerned with? Evan Harwich reserved his determination for what he knew was coming. The slow wearing down of stamina. Water he and Paul had a little of. And more could be reclaimed from the thin, dry atmosphere. It collected in the bottoms of oxygen bottles, when they were pumped full, condensed by compression. A few precious drops. You could drink it out after each bottle was emptied of air. Just about enough water to sustain life.