* * *

My first impression was of vast space, a colossal reach of space that stretched far away before us, and into which our ship was racing on. And then I saw, with sudden awe and wonder, that this vast space was not the unlimited, unbounded space we were accustomed to, but was limited, was bounded, bounded by a colossal sheet of flowing flame that hemmed it in all directions. Above and below and before and behind us stretched this mighty wall of flame, a gigantic shell of fire that enclosed within itself the vast space in which our cruiser raced, a space large enough to hold within it a dozen solar systems like my own. Stunned, we gazed out into that mighty flame-bounded space, and then I flung out a hand toward it in sudden comprehension.

"We're inside the nebula!" I cried. "It's hollow! This vast open space lies at its heart, and those currents carried us down into it!"

For I saw now that this was the explanation. Unsuspected by any in the Galaxy the mighty nebula was hollow, its gigantic globe of flaming gas holding at its heart this mighty empty space, a space mighty in extent to our eyes, but small compared to the thickness of the great shell of fire that enclosed it. And down through that fire, that vast ocean of flame, the currents of the nebula had brought us, from its outer surface, down into this great space at its heart of which none had ever dreamed, and into which we had been the first in all the Galaxy to penetrate.

While we gazed across it, stunned, our cruiser was racing on into this vast hollow, away from the wall of flame behind us from which we had just emerged. And now, as we flashed on, Sar Than cried out too and pointed ahead. There, standing out black against the encircling walls of fire in the distance, was a small round spot, a spot that was growing to a black globe as we hurtled on toward it, a globe that hung motionless at the center of this mighty space, here at the nebula's heart. We were racing straight into the great cavity toward it and now there came a low exclamation from Jor Dahat, beside me, as his eyes took in the great globe ahead.

"A planet!" he whispered. "A planet here-within the nebula!"

My own eyes were fixed upon it, and slowly I nodded, but made no other answer as we flashed on toward the object of our attention, the black sphere ahead. And now as we swept on we saw that it was a sphere of truly titanic dimensions, larger by far than any of the Galaxy's countless worlds, and that as it hung there, at the nebula's heart, it was slowly revolving, spinning, as fast or faster than the nebula itself. Black and mighty it hung there, while all around it, millions of miles from it, there flamed the nebula's encircling fires. On and on we raced toward it, and for all those minutes of flashing flight none of us spoke, and there was no sound in the pilot room but the throbbing drone of the generators below. I think that we all felt instinctively that in the grim, colossal globe ahead lay the answer to what we had come to solve, and as we hurtled on toward it we watched it broadening before us in tense silence.

Larger and larger it was becoming, larger until its great black circle filled half the heavens before us. By then I had decreased our speed to a fraction of its former figure, and as we swept in toward the giant world I lessened it still further. Slowly, ever more slowly we moved, and now were circling above the great black planet, were beginning to drop cautiously down toward it. Eagerly we watched as the mighty world's surface changed from convex to concave, and as we dropped on we saw the needle of our atmosphere-pressure dial moving steadily forward, to show that this strange world had air, at least. Then all else was forgotten as our eyes took in the scene below.

I think that we had all half expected to see some evidence of life and civilization on this strange world, some building or group of buildings, at least. But there was none such. Beneath us lay only a smooth black plain, extending from horizon to horizon, devoid of hill or stream or valley, in so far as we could see, unnaturally smooth and level. And as we dropped nearer, ever nearer, the surprise we felt rapidly intensified, until when at last we hung motionless a hundred feet above the surface of this world exclamations of utter astonishment broke from us. For seen thus near, the surface of this mighty planet was as utterly smooth and level as it had seemed from high above, a black, gleaming plain without an inch-high elevation or depression, an inconceivably strange smooth expanse of black metal, that stretched evenly away in every direction to the horizon, smoothly covering this colossal world.

We looked at each other, a little helplessly, then down again toward the smooth and gleaming surface below. In that surface was no visible opening, no sign of joint or crack, even, nothing but the smooth blank metal. Then with sudden resolution I thrust forward the levers in my hands, sent our cruiser racing low across the surface of the giant, metal sheathed planet, while we gazed intently across that surface in search of any sign that might explain the enigma of its existence. On we sped, while beneath us flashed back the smooth metal plain, mile after endless mile. Then, gazing ahead, my eyes suddenly narrowed and I raised a pointing hand. For there, far ahead, I had glimpsed an opening in the gleaming surface, a round black opening that was resolving itself into a vast circular pit as our cruiser raced on toward it.