"Tell them how we worked that out," said Ferrati, who had begun to grow again the short black beard that Burl remembered he had worn on his famous expeditions. "Well," said Haines, drawing the word out to build up suspense, "did you know that the station in the Andes, the one you cracked open, was built at least thirty years ago? And never put into operation in all that time?" Burl was surprised. "Why ... I hadn't thought of it--but it could have been. That valley was so isolated and deserted, probably nobody would ever have spotted it. "Right," Haines added, "and our investigation team studied the remains, the foundations, the layout, and we're sure it's been there at least three decades. That's one clue. "The second clue was the relative flimsiness of the walls. The builders hadn't expected us to be able to blow them up. They were some sort of quick construction--a plastic, strong, but not able to hold up against blasting powder, let alone real heavy bombs or A-bombs. "Now why was that? And the third clue, why didn't they have a repair system available, or at least some sort of automatic antiaircraft defense?" Burl looked at Ferrati. The latter was watching him shrewdly to see if he could figure it out. "The builders didn't expect an air attack," said Burl slowly, "because of the air disturbances. They did not know we would have a Moon base that could spot their location. Hence they figured that our civilization would remain as it was thirty years ago. We wouldn't have been able to spot the location at that time, because it required outer-space observation. It might have taken us several years of tramping around to locate it." "And the lack of a strong permanent construction? After all, a concrete and steel-enforced embankment, which any military force on Earth could have put there, would have balked your dynamite attack," probed Haines. "That means they didn't have the time or the means to make such a construction. They must have had a single ship with the kind of equipment that could lay out a quick base in the shortest time!" said Burl. "Right!" snapped Haines. "The Sun-tap must have been built by a relatively small team, which probably came in a single explorer ship. The ship was equipped with automatic factory machinery that could turn out an adequate base for an uninhabited planet, an airless moon, and so on--but they didn't have the stuff for a fortified base--and they didn't have the manpower to build it."