On and on it moved, its little pinkish feet pattering almost silently on the oxidized metal surface of the rail. Its sensitive ears picked up the movements and the squeals of other rats, but it paid them no heed. Several times, it met other rats on the rail, but most of them sensed the alienness of _this_ rat and scuttled out of its way. Once, it met a rat who did not give way. Hungry, perhaps, or perhaps merely yielding to the paranoid fury that was a normal component of the rattish mind, it squealed its defiance to the rat that was not a rat. It advanced, baring its teeth. The rat that was not a rat became suddenly motionless, its sharp rodent's nose pointed directly at the enemy. There came a noise, a tiny popping hiss, like that of a very small drop of water striking hot metal. From the left nostril of the not-rat, a tiny glasslike needle snapped out at bullet speed. It struck the advancing rat in the center of the pink tongue that was visible in the open mouth. Then the not-rat scuttled backwards faster than any rat could have moved. For a second, the real rat hesitated, and it may be that the realization penetrated into its dim brain that rats did not fight this way. Then, as the tiny needle dissolved in its bloodstream, it closed its eyes and collapsed, rolling limply off the rail. The rat might come to before it was found and devoured by its fellows--or it might not. The not-rat moved on, not caring either way. The human intelligence that looked out from the eyes of the not-rat was only concerned with getting to the Nipe. * * * * * "That's how we found the Nipe," Colonel Mannheim said, "and that's how we keep tabs on him now. We have over seven hundred of these remote-controlled robots hidden in strategic spots in those tunnels now, but it took time to get everything set up this way. Now, we can follow the Nipe wherever he goes, so long as he stays in the tunnels. If he went out through an open air exit, we could have him followed by bird-robots but--" He shrugged wryly. "I'm afraid the underwater problem still has us stumped. We can't get the carrier wave for the remote-control impulses to go far underwater." "How do you get your carrier wave underground to those tunnels?" Stanton asked. The colonel grinned widely. "One of the boys dreamed up a real cute gimmick. The rails themselves act as antenna for the broadcaster, and the rat's tail is the pickup antenna. As long as the rat is crawling right on the rail, only a microscopic amount of power is needed for control, not enough for the Nipe to pick up with his instruments. Each rat carries its own battery for motive power, and there are old copper power cables down there that we can send direct current through to recharge the batteries. And, when we need them, the copper cables can be used as antennas. It took us quite a while to work the system out."