There was an ominous rumbling sound, then a terrifying feeling of the vibration of disintegration.
They bolted for the opening even as the still-standing masonry which formed it began to topple. The rumbling increased to thunder-volume, and the earth outside the collapsing ruin quaked beneath their running feet. When they finally stopped at a safe distance, their night-lamps showed only a slowly rising cloud of pumice and dust.
"How often," Markten said, when it at last was over, "do you forget the fundamentals of your basic training?"
"I—"
"It's done now. But the contents of whatever lower levels there may have been are lost to us for good. Nothing could have survived that. And we have never built a digging machine. There probably was nothing, anyway, but next time—"
Then Markten saw the book in his aide's hand. The look of disappointment on his features changed suddenly to one of disbelief, then to amazement.
"At least I saved this! It has diagrams, Markten! The cave-in I caused destroyed three other books, but they had no drawings in them at all. Here. See if you can understand the title."
"Let's get to the laboratory compartment of the plane, where we can see something! Great electrons, boy, what made you hold this back?"
Under the powerful lamps in the lab compartment of their aircraft, Markten and the finder of the book puzzled over the three words on its cover and fly-leaf.