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!