“I wanted you to see it for yourself before I said anything. That picture is now thirty miles in diameter, and those squares are two or three miles on a side. You’ll notice that the vertical lines converge and the horizontal ones are bent into arcs. We’re looking at part of an enormous structure of concentric rings; the centre must lie many miles to the north, probably in the region of Cambridge. How much further it extends in the other direction we can only guess.”

“But what is it, for heaven’s sake?”

“Well, it’s clearly artificial.”

“That’s ridiculous! Fifteen miles down!”

The Professor pointed to the screen again. “God knows I’ve done my best,” he said, “but I can’t convince myself that Nature could make anything like that.”

I had nothing to say, and presently he continued: “I discovered it three days ago, when I was trying to find the maximum range of the equipment. I can go deeper than this, and I rather think that the structure we can see is so dense that it won’t transmit my radiations any further.

“I’ve tried a dozen theories, but in the end I keep returning to one. We know that the pressure down there must be eight or nine thousand atmospheres, and the temperature must be high enough to melt rock. But normal matter is still almost empty space. Suppose that there is life down there—not organic life, of course, but life based on partially condensed matter, matter in which the electron shells are few or altogether missing. Do you see what I mean? To such creatures, even the rock fifteen miles down would offer no more resistance than water—and we and all our world would be as tenuous as ghosts.”

“Then that thing we can see—”

“Is a city, or its equivalent. You’ve seen its size, so you can judge for yourself the civilization that must have built it. All the world we know—our oceans and continents and mountains—is nothing more than a film of mist surrounding something beyond our comprehension.”