It was. The poles of Callisto were actually two continent-sized islands of shell. Dry, mudlike stuff, hard as rock, floating on the endless seas of the semiliquid planet.
The station, a ringed setup quite like the one on Earth, stood in the geographic center of the south pole. The Magellan hovered over it while a landing party went down in the four-man rocket plane.
Clyde, Haines and Burl were the landing party. This time, only Burl entered the station after a hole had been blasted in the outer shell. While the redheaded astronomer took samples and made observations, Haines kept watch. Nobody knew what type of defense awaited them here.
Burl found the controls easily enough. He was afraid that he might have lost his physical charge, but it was not so. The controls functioned, the Sun-tap station died. The effect was not very noticeable, for Callisto was already far from the Sun and the thin atmosphere could not diminish the dark sky of outer space. What the great masts caught must have been only the relay from other stations—or perhaps the invisible rays of the distant Sun.
Burl saw no reason to linger, and the three of them gathered up their equipment. As they started back toward the rocket plane, they heard an ominous rumble in the ground.
A sudden spurt of blazing gas shot up from the center of the station. "Duck!" yelled Haines, and they fell flat on the ground. Burl held his hands protectively over his head, as an explosion shook the building.
There was no rain of rocks. Whatever the blast, Callisto's gravity was too weak to attract the debris that flew high above the station.
"It was an atomic explosion!" Haines shouted into his helmet mike. "They mined the station. Run for it!"
They raced for the rocket plane. As they ran, Burl felt the ground quiver beneath him, and huge cracks began to spread, rippling through the hard ground.
They reached the plane and piled in. Russ took off just as the surface cracked open in a thousand places like an ice sheet breaking in an Arctic thaw.