Burke said abruptly, "Keller, the stone being freezing cold while the air's warm means that this place has been heated up lately. Heat's been poured into it. Within hours!" Keller considered. Then he shook his head. "Not heat. Warmed air." Burke went scowling onward. He followed, actually, the only route that was open. Other ways were cut off by doors which refused to open. Sandy, beside him, noted the floor. It was stone like the walls and ceiling. But it was worn. There were slight inequalities in it, beginning a foot or so from the walls. Sandy envisioned thousands of feet moving about these resonant corridors for hundreds or thousands of years in order to wear away the solid stone in this fashion. She felt age about her--incredible age reaching back to time past imagining, while the occupants of this hollow world swarmed about its interior. Doing what? Burke considered other things. There were the ten-foot metal spheres, ranged by hundreds in what might be a magazine below. There were the squat and ugly metal monsters which seemed definitely menacing to somebody or something. There were the metal frameworks like bunks. There was no rust, here, which could be accounted for if Keller happened to be right and warmed air had been released lately in corridors which before--for ten thousand years or more--had contained only the vacuum of space. And there were those rooms which could be mess-halls. These items were subject matter for thought. But if what they hinted at was true, there must be other specialized compartments elsewhere. There must be storerooms for food for those who managed the guns--if they were guns--and the spheres, and lived in the bunk-rooms and ate in the mess-halls. There'd be storerooms for equipment and supplies of all sorts. And again, if Keller were right about the air, there must be enormous pressure-tanks which had held the asteroid's atmosphere under high pressure for millennia, only to warm it and release it within the hour so that those who came by ship could use it. An old phrase occurred to Burke. "A mystery wrapped in an enigma." It applied to these discoveries. Plainly the release of air had been done without the command of any living creature. There could be none here! As plainly, the signals from space had been begun without the interposition of life. The transmitter which still senselessly flung its message to Earth was a robot. The operation of the ship-lock, the warming of air, the lighting of the ship-lock and the corridors--all had been accomplished by machinery, obeying orders given to the transmitter first by some unguessable stimulus.