“Well, Professor,” I said, “I congratulate you. It’s a wonderful achievement. But we seem to have reached the core now. I don’t suppose there’ll be any change from here to the centre.”

He smiled a little wryly. “Go on,” he said. “You haven’t finished yet.”

There was something in his voice that puzzled and alarmed me. I looked at him intently for a moment; his features were just visible in the blue-green glow of the cathode-ray tube.

“How far down can this thing go?” I asked, as the interminable descent started again.

“Fifteen miles,” he said shortly. I wondered how he knew, for the last feature I had seen at all clearly was only eight miles down. But I continued the long fall through the rock, the scanner turning more and more slowly now, until it took almost five minutes to make a complete revolution. Behind me I could hear the Professor breathing heavily, and once the back of my chair gave a crack as his fingers gripped it.

Then, suddenly, very faint markings began to reappear on the screen. I leaned forward eagerly, wondering if this was the first glimpse of the world’s iron core. With agonizing slowness the scanner turned through a right angle, then another. And then…

I leaped suddenly out of my chair, cried “My God!” and turned to face the Professor. Only once before in my life had I received such an intellectual shock—fifteen years ago, when I had accidentally turned on the radio and heard the fall of the first atomic bomb. That had been unexpected, but this was inconceivable. For on the screen had appeared a grid of faint lines, crossing and recrossing to form a perfectly symmetrical lattice.

I know that I said nothing for many minutes, for the scanner made a complete revolution while I stood frozen with surprise. Then the Professor spoke in a soft, unnaturally calm voice.