While not artificial, the Hill had been altered markedly by the Triplanetary engineers who had built into it the headquarters of the Secret Service. Its mile-wide top was a jointless expanse of gray armor steel; the steep, smooth surface of the truncated cone was a continuation of the same immensely thick sheet of metal. No known vehicle could climb that smooth, hard, forbidding slope of steel; no known projectile could mar that armor; no known craft could even approach the Hill without detection. Could not approach it at all, in fact, for it was constantly inclosed in a vast hemisphere of lambent violet flame through which neither material substance nor destructive ray could pass. As the _Silver Sliver_, crawling along at a bare three-hundred miles an hour, approached that transparent, brilliantly violet wall of destruction, a violet light filled her control room and as suddenly went out; flashing on and off again and again. "Giving us the once-over, eh?" Cleveland asked. "That is something new, isn't it, Fred?" "Yes, it's a high-powered ultra-wave spy," Rodenbush returned. "The light is simply a warning, which can be carried if desired. It can also carry voice and vision...." "Like this," Samms' voice interrupted from the powerful dynamic speaker upon the pilots' panel and his clear-cut face appeared upon the television screen. "I don't suppose Fred thought to mention it, but this is one of his inventions of the last few days. We are just trying it out on you. It doesn't mean a thing though, as far as the _Sliver_ is concerned. Come ahead!" A circular opening appeared in the wall of force, an opening which disappeared as soon as the plane had darted through it; and at the same time her landing-cradle rose into the air through a great trap-door. Slowly and gracefully the space-plane settled downward into that cushioned embrace. Then cradle and nestled _Sliver_ sank from view and, turning smoothly upon mighty trunnions, the plug of armor drove solidly back into its place in the metal pavement of the mountain's lofty summit. The cradle-elevator dropped rapidly, coming to rest many levels down in the heart of the Hill, and Cleveland and Rodebush leaped lightly out of their transport, through her still hot outer walls. A door opened before them and they found themselves in a large room of full daylight illumination; the anteroom of the private office of Virgil Samms. Chiefs of Departments sat at their desks, concentrated upon problems or at ease, according to the demands of the moment; televisotypes and recorders flashed busily but silently; calmly efficient men and women went wontedly about the all-embracing business of Triplanetary's space-pervading Secret Service. "Right of way, Norma?" Rodebush paused briefly before the desk of the Chief's private secretary; but even before he had spoken she had pressed a button and the door behind her swung wide. "You two do not need to be announced," the attractive young woman smiled. "Go right in."