The first thing that I noted, gazing downward, was that the space beneath the giant metal roof of Neptune was a great one, since it was a full mile from the opening in that roof to the surface of the world far below. Gazing down toward that surface, seeing at last the true surface of Neptune lying in the brighter light that existed in some strange way beneath the gigantic metal roof, we gasped. For upon that surface there loomed countless strange structures such as we had never seen before. Rectangular in shape were those structures, with straight black walls, of great size but seeming rather low in height, and they were without exception roofless! In them we could dimly make out from our great height the gleaming shapes of what seemed huge machines of one sort or another, but could not at that height see whether living beings of any sort moved among the structures. The great circle of the world beneath that we could see, hanging above the opening, was completely covered with these structures, the black walls of one roofless building being surrounded on all sides by the walls of others, there being no streets or open spaces whatever between them! It was as though, indeed, all the surface of the great world beneath had been divided into great compartments by a great checkerboard arrangement of intersecting black walls!
Marlin's eyes were gleaming with excitement as he gazed down. "The city of the creatures of Neptune!" he breathed, as in awe we four stared down. "The city of Neptune that lies beneath the colossal roof, and that must hold somewhere that which we have come to seek!"
"You're going to venture down into this city—down under the great roof?" I asked, and he nodded.
"We must, Hunt, to find the giant force-ray's source. But stand ready to flash the space-flier back upward—for if we're discovered by whatever beings inhabit this strange world, I think we'll get short shrift!"
A moment we paused there, and then as my hands moved upon the switch-controls, decreasing the power of the force-ray that held us upward the space-flier was sinking smoothly and slowly downward, down through the great opening! Tensely and with fascinated interest we gazed about now as we sank into the great space that lay beneath the huge metal roof. That space was brighter-lit than above the roof, we saw, and as we turned a moment to glance upward we saw that looking upwards, the roof was perfectly transparent! Dark, opaque metal when seen from above, it was almost invisible in its transparency when seen from below! And, seeing that, we understood the great roof's purpose. It had been constructed and placed above all Neptune, encircling the great planet and enclosing it, to retain that planet's heat as much as possible. For it was apparent that heat and light radiations or vibrations could not pass up through the metal of the roof from beneath, making it appear black and perfectly opaque from above, but could pass freely down through it from above, making it appear almost perfectly transparent from below!
Even as we grasped the wonder of that, though, we had forgotten it, in the greater wonder of the things that lay now before our eyes. For as we sank down in our space-flier into the great space beneath that roof we could see the surface of great Neptune itself, stretching far away beneath that mighty enclosing shield above it, and covered to the horizons by the strange rectangular and roofless structures such as we had already seen. These were formed, indeed, by smooth black walls of some two hundred feet in height that ran in straight lines in checkerboard arrangement across all the surface of this huge planet, apparently, forming upon all its surface, without streets or parks or openings of any kind, a vast city of rectangular compartments, large and small! A titanic streetless city that covered apparently all the surface of giant Neptune!
But most wonderful of all the things that lay before us in that moment was the fact that nowhere about us could we see any sign of supporting pillars or piers for the giant roof that stretched far above us! For though we could gaze far away to the distant horizons of this great world, we could find no single support for that huge metal roof that apparently covered all the great planet, and whose weight must have been incalculable! And another feature of the giant roof puzzled us. It puzzled us to see the great openings in it like that down which we had come, great circular openings which we could see in it here and there at great distances from each other. Those openings were provided on their under-side with great sliding shutters for closing them tightly, yet all were open! Why should they be open, we silently asked ourselves, if the purpose of the roof was to retain Neptune's heat within that roof? For the existence of those great and unclosed openings in the roof must surely be defeating that purpose, for our outside-temperature dial recorded the same zero temperature as prevailed above the roof!
Yet even these strange things could not wholly draw our interest and attention from the strange compartment-city beneath, as our space-flier sank toward it. We were within a few hundred feet of it, now, and as we dropped nearer, Marlin and Whitely and Randall staring eagerly down beside me, my hands were tense upon the switch-controls, ready to send our flier leaping instantly upward. For if the beings of the city beneath, whatever their nature, caught sight of us, we could expect nothing but instant attack. So that a tenseness held all of us as our great flier's faceted polyhedron dropped on through the pale light beneath the great roof toward the black-walled, checkerboard-like city that stretched across the surface of the great world beneath us. And now, as we sank lower, our eyes were making out ever more clearly the details of that amazing city.
The rectangular black-walled compartments held, as we had half-realized from above, various strange-shaped mechanisms and objects which we could even now only vaguely discern. We could see clearly, though, that here and there across all the vast city's compartmented surface there stood giant metal globes, each a hundred feet in diameter and each occupying a square compartment of its own. There seemed hundreds of these great gleaming globes, scattered here and there in compartments across the city's surface as far as we could see, though their purpose was then quite incomprehensible to us. But as we sank lower still, ever more cautiously, it was not the globes or the compartments' contents that held our attention so much as the astounding, stupefying fact that now was thrust upon us—namely that in all the gigantic compartmented city, in all its strange great black-walled rectangular and roofless enclosures, there moved no living being!