“I sure hope the gas stays where it belongs while we’re down there!” Steve said and swallowed the lump of fear in his throat.
They turned their attention to Jupiter. It looked even more like a beach ball now with its stripes of beautiful colors. Mr. Shannon said the bands were floating ice bergs of the poisonous gases he was talking about.
“No ship can land on Jupiter,” he said. “Its gravity would crush a spaceman flat. Gravity pull is much stronger on the larger planets, you know. Jupiter’s atmosphere is many thousands of miles deep. Raging storms are going on beneath it all the time.”
“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?”