"There it is, Boss, right there."
A soft glow, like moonlight, filtered through the interstices of the crucible, dimly illuminating the cavern. Dust moved beneath their feet, dust that had not been disturbed for ages. Oscar sneezed.
A heavy, cup-like crucible with cracked walls that had been several feet thick ... in the center was a softly glowing ball.... Andy bent over it.... Radium.... There was no doubt.... But....
"Hell," he said, his jaw dropping. "Hell...."
His eyes caught the heavy outlines in the dust on the floor. He stirred it with his toe.
"Intelligence," he muttered. "Intelligence was here, in this cavern perhaps a hundred centuries ago. The crucible is lead, incredibly old. Perhaps part of it was once radium. It was the heart of some kind of an engine, some method of releasing energy, possibly hundreds of thousands of years ago. Look! You can see in the dust where other metals, which formed a framework, have oxidized...."
The girl said nothing, and Oscar, for once was silent.
"Once there was intelligent life on Io. It built this, and left it for some reason that we can't even guess at."
Frieda stared at the glowing metal, moved back.