He had been going ahead of her in the narrow tunnel. Now it widened and they were able to walk side by side. There seemed to be no end to it. But then they came to a sturdy-looking door, padlocked. "That's that," Don said. "That's that nothing," she said. "Break it down." He laughed. "You flatter me. Come on back." "Don't you think this is at all peculiar? A tunnel starting under an abandoned grandstand, running all this way and ending in a locked door?" "Maybe this was a station on the underground railway. It looks old enough." "We're going through that door." She opened her purse and took out a key ring. On it was an extensive collection of keys. Eventually she found one that opened the padlock. "Well!" he said. "Who taught you _that_?" "Open the door." The corridor beyond the door was lined--walls, ceiling and floor--with a silvery metal. It continued east a hundred yards or so, swung north and then went east again, widening all the time. It ended in a great room whose far wall was glass or some equally transparent substance. The room was a huge observatory at the end of Superior but below its rim. They could look down from it, not without a touch of nausea, to the Earth four miles below. Don, thinking of the surface of Superior above, thought it was as if they were looking out of the gondola slung beneath a dirigible. Or from one of the lower portholes in a giant flying saucer. V There were clouds below that occasionally hid the Earth from sight. For a minute or more they gazed in silence at the magnificent view. "This wasn't built in a day," Jen Jervis said at last. "I should say not," Don agreed. "Millions of years." She looked at him sharply. "I wasn't talking about the age of the Earth. I mean this room--this lookout post--whatever it is." He grinned at her. "I agree with you there, too. I'm really a very agreeable fellow, Miss Jervis. Obviously, whoever built it knew well in advance that Superior was going to take off. They also knew how much of it was going up and exactly where this would have to be built so it would be at the edge." "Under the edge, you mean, with a downward view." "That's right. From a distance I'd say Superior looked as if someone had cut the end off an orange. The flat part--where the cut was made--is the surface and we're looking out from a piece of the convex skin." "You put things so simply, Mr. Cort, that even a child could understand," she said acidly. "Thank you," he said complacently. He had remembered that whoever was listening in for Military Intelligence through the tiny radio under his shirt could have only a vague idea of what was going on. Any little word pictures he could supply, therefore, would help them understand. He had to risk the fact that his companion might think him a bit of an idiot.