“Ooo!” Sue gasped. “I guess we’re close enough to it then!” Other wonders of the sky were the round beacons of Jupiter’s other moons, three of which were about the same size as Callisto. They hung like bright searchlights in the starry heavens. The men at the mining place greeted the Shannons warmly. They had not seen anyone from Earth for so long that they had grown very lonely. The chief mining engineer said he would be glad to take the visitors on an underground tour. His name was Dr. Harding. He was plump and short and wore black-rimmed glasses inside his space helmet. He led them into an elevator and it sank into the darkness. Steve remembered about the poisonous gases that crept about underground and it made him shiver to think about it. Dr. Harding watched Bud hopping around uncomfortably inside his small space cage. “Do you remember, Mr. Shannon,” he asked over his suit radio, “when they used to use canary birds in mines to warn about leaking gas? The birds would notice it first and give the miners time to get out.” “I’ve read about that, Dr. Harding,” said Mr. Shannon. “Now we have automatic warning machines in the tunnels to do that,” the chief engineer told Sue and Steve. Deeper and deeper below the soil of Callisto the elevator sank. At last the cage reached the bottom, and the riders found themselves in a large cavern. There were machines and men all about, working busily. Tracks led off into tunnels and ore cars were running on them. Some were going empty into the tunnels while others were coming out full of rock and gravel. “The magna is separated from the rock in that big machine over there,” Dr. Harding explained. “Want to ride an ore car into one of the tunnels?” “Sure!” Steve spoke up. “The mine is air-conditioned,” the chief engineer said, “so we can take off our helmets.” This done, Steve let Bud out of his cage. The little bird hopped up on his gloved finger, saying, “Rocket away!” several times. His two-word language seemed to do for everything. One worker controlled all the cars at a main switch in the middle of the cavern. The Shannons and their guide climbed into an empty ore car and it rolled into a tunnel. Glistening dark rock crowded in on Sue and Steve from all sides. Steve hoped the walls were strong enough so they would not come crashing down on their heads! There were lights along the way to help brighten the gloom. After clicking along like a trolley for awhile, the car came to the end of the line. It was a large room with more machines and workmen. The men were digging magna ore out of the wall with drills. As Dr. Harding explained about the work, Bud began flitting about as though sight-seeing on his own. He was shy of the workers at first, but then made friends with them. He spoke to them with his favorite two words and the men laughed in great fun to hear him. Then a few minutes later, Bud began acting queerly. He flew back to Steve’s finger and started wobbling as though dizzy.