Rawson's reply was not an answer to Gor so much as it was an argument with himself. "Heavy," he said. "Specific gravity beyond anything we've ever known. Osmium, the heaviest substance we have, would be light as a feather compared to this. But wait. This Grah, as you call it, falls downward, but that means it falls toward the outside of the earth. With us it would be light--light! And Oro would be heavy. New substance--new matter! One feels only the attraction of our normal gravitation; the other doesn't react to that at all, but is driven outward with tremendous force by counter-gravitation, the repulsion of this Central Sun. You've used it cleverly, but we'd have done more with it up on top." * * * * * He was lost in thought for some minutes, muttering figures and calculations half aloud. "Two thousand miles from the Central Sun to us; two thousand more through the solid earth. And if that repelling force follows Newtonian laws it will decrease as the square.... But, coming down from up on top, normal gravity would decrease directly as the distance!" He made scratches with one small stone upon a larger one in lieu of paper and pencil, but, to his listeners, his muttered words could have meant nothing. "Around six seventy-six hundred and seventy miles to the neutral zone, the Zone of Fire. And a column of water--it would carry on by, plug the shaft, check the back-pressure, and then...." For the first time since that night when the mole-men had poured out into the crater, his eyes were alight with hope, though his face seemed tense and grim. Then the lines about his lips relaxed; he smiled at Loah. "I would like to investigate this under-world," he said, "--not very far down. Will you take me?" The girl's adventurous spirit had led her on many exploring trips in that subterranean world. She laughed happily when Rawson told her what he wanted. "But, yes," she said; "of course I know such a place." And from some two or three miles below, after anchoring the _jana_ securely, she led him through a winding tunnel where he knew he was steadily climbing. * * * * * It was a wide corridor that they followed, where the walls came together high above their heads; he could hardly see where they met by the light of Loah's torch. Now and then there were lateral passages, but they were narrow, hardly more than cracks; and Rawson, looking into them, nodded his head with satisfaction. Occasionally his footsteps rang hollowly on the stone, and he knew that the floor was thin between this and other caverns below. "What an old honeycomb it is!" he exclaimed. "And we had it all figured as being solid. The weight is all here, of course, but it's concentrated in that red stuff down near the neutral zone. But anyway, Loah has shown me just what I wanted."