The animal remained where it was. They climbed over a rock slide and faced a wide bronze door protected by a concrete foyer. Out a way from the door was-- "Look, Ken--that's been a recent campfire!" He whipped the blade from its sheath. "C'mon, kitten--get that knife out!" He vaulted the ashes. A six-inch square was cut deeply in the dense metal. Ken poised his knife over a slot, and as Carol plunged her blade into the wall he rammed his home to the guard. With a squeak and a sigh the door, terraced like a vault portal, swung outward slowly. Ken grabbed a recessed knob to hurry it up. Lights flashed inside, flooding a man-changed interior. He leaped across the raised threshold, dragging Carol with him, swung the door shut and shot home two great bolts on its inner surface. On a rack just beside the door was an automatic rifle, ready for instant use. The psychologists had not known about the campfire, but they had planned for the possibility of a hostile builder. Ken and Carol looked about the first of the labyrinthine caverns. Squared walls were lined solidly with glass-enclosed bookshelves stretching as far as they could see. Crowding the floor were machines, cabinets of tools, implements, instruments, weapons and medical and surgical supplies. They moved to stand before a large video screen set near the door. Ken flipped the single toggle below it. A scene grew, showing a white-haired army colonel seated behind a desk, facing them. "Ken--it's Dr. Halsey," Carol whispered. "_Was_ Dr. Halsey," said Ken heavily. "I used to wonder if we had the same instructors." The officer's lips moved. "Hello, Ken and Carol. I've been selected to make this film to greet you, and I know both of you will return to see it." His eyebrows lifted in the quizzical expression they knew well. "I'm going to rattle off a lot of explanations and suggestions, but I imagine the first thing you'll want to know is how all the things you've seen could have happened so quickly. And knowing that, will clarify the rest. "You remember the experiments the Air Force made, sending small animals above the stratosphere. By means of controlled diets and more complicated devices you'll find explained in a book, we learned that these animals were not subjectively experiencing the time-span they should have aloft. In effect, they were aging hardly at all away from gravity--the farther away, the less aging. "We got some fairly accurate figures on the time-distance ratios. Briefly, assuming you held to your course until you were recalled, you can figure that an average of ten years has elapsed on the earth's surface for every hour you were in space." Ken muttered, "We were actually out of atmosphere about twenty-four hours. That would make it the year--" "About 2200," finished Carol breathlessly.