When we entered the hut containing the display equipment, the Professor was strangely silent. He switched on the transmitter, and even though it was a hundred yards away I could feel an uncomfortable tingling. Then the cathode-ray tube lit up and the slowly revolving time-base drew the picture I had seen so often before. Now, however, the definition was much improved owing to the increased power and sensitivity of the equipment. I adjusted the depth control and focused on the Underground, which was clearly visible as a dark lane across the faintly luminous screen. While I was watching, it suddenly seemed to fill with mist and I knew that a train was going through.
Presently I continued the descent. Although I had watched this picture many times before, it was always uncanny to see great luminous masses floating towards me and to know that they were buried rocks—perhaps the debris from the glaciers of fifty thousand years ago. Dr Clayton had worked out a chart so that we could identify the various strata as they were passed, and presently I saw that I was through the alluvial soil and entering the great clay saucer which traps and holds the city’s artesian water. Soon that too was passed, and I was dropping down through the bedrock almost a mile below the surface.
The picture was still clear and bright, though there was little to see, for there were now few changes in the ground structure. The pressure was already rising to a thousand atmospheres; soon it would be impossible for any cavity to remain open, for the rock itself would begin to flow. Mile after mile I sank, but only a pale mist floated on the screen, broken sometimes when echoes were returned from pockets or lodes of denser material. They became fewer and fewer as the depth increased—or else they were now so small that they could no longer be seen.
The scale of the picture was, of course, continually expanding. It was now many miles from side to side, and I felt like an airman looking down upon an unbroken cloud ceiling from an enormous height. For a moment a sense of vertigo seized me as I thought of the abyss into which I was gazing. I do not think that the world will ever seem quite solid to me again.
At a depth of nearly ten miles I stopped and looked at the Professor. There had been no alteration for some time, and I knew that the rock must now be compressed into a featureless, homogeneous mass. I did a quick mental calculation and shuddered as I realized that the pressure must be at least thirty tons to the square inch. The scanner was revolving very slowly now, for the feeble echoes were taking many seconds to struggle back from the depths.
“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.