Cliff mused. "Doesn't seem attenuated enough though . . ."
He shrugged and turned to the borers.
"All right; finish the job," he said. "Use full range this time and plow right through."
THIS time the beam incorporated an area wide enough to permit of the entire borer machine following it through. As before it took it thirty minutes to nose its juggernaut way through the wall, which was all of twenty-five to thirty feet in thickness. Once beyond the barrier engineers stood sniffing the stale, musty air and gazing round in the glare of the searchlights.
They were within a colossal artificially bored cavern, filled with an extraordinary number of gray metal balls dotted about in various directions. Some were large and some small, but all were bolted and riveted immovably to tripod stands of metal. Right and left they went, round the natural curve of the cavern out of sight. In the cavern's center was yet another ball of metal, gray like the smaller balls, and apparently a kind of master ball. The distance to the major ball was perhaps two miles. How far the cavern itself really extended was lost in darkness.
The air seemed to be coming from a source in the cavern hidden by the major ball.
Cliff climbed down from the borer and went to the nearest ball, stood looking at it perplexedly. At last he turned to the others and held up his hand for silence.
In a moment it was clear that the little ball was whirring mysteriously like a spring uncoiling.
"Machinery!" Richardson ejaculated.
Silence fell on the party again as there came a new sound through the heavy silence—a solemn, deliberate ticking like that of a giant grandfather clock.
It went on steadily and Cliff consulted his watch.
"Something is ticking at exactly three second intervals," he proclaimed finally. "And it has only just started. . . . Looks like we have stumbled onto something, boys."
Val said slowly, "The ticking comes from that giant ball there. Let's take a look at it."
They mounted the borer again and drove forward the intervening distance. The progress of the journey made the ticking all the more audible, until by the time they had reached the giant ball itself it was a solemn reverberation that boomed along the floor.
Tick . . . Tock. Tick . . . Tock.
"Time bomb?" Vale suggested laconically.
"Quit clowning," Cliff snapped impatiently. "It's pretty plain we started the works going by coming in here. Nothing happened until we went over to look at that smaller ball. Somehow I don't like it. There's a deliberation about that ticking that’s kind of ominous."
"Yeah . . ." Val meditated. He said, "Suppose before we start forming opinions we look around a bit? This air mystery, for instance . . ."
CHAPTER II
Invisible Enemy
AT the rear of the giant ball they discovered the reason for the air supply and its un-Martian density. A titanic vent sunken into the floor, and presumably communicating by a complicated shafting system to the other side of Mars itself—right through to the surface—was covered with a massive
