"Well, at first we didn't know for sure what he was up to; we weren't even sure he was actually down in those tunnels. But we suspected that if he was he'd have alarms set all over the place—perhaps even alarms of types we couldn't recognize. But we had to take that chance. We had to watch him."
He walked over to the nearby table and opened a box some twelve inches long and five-by-five inches in cross-section.
"See this?" he said, as he took a furry object from the box.
It looked like a large rat. Dead, stiff, unmoving.
"Our spy," said Colonel Mannheim.
The rat moved along the rusted steel rail that ran the length of the huge tunnel. To a human being, the tunnel would have seemed to be in utter darkness, but the little eyes of the rat saw the surroundings as faintly luminescent, glowing from the infra-red radiations given out by the internal warmth of the cement and steel. The main source of the radiations was from above, where the heat of the sun and the warmth from the energy sources in the buildings on the surface seeped through the roof of the tunnel. But here and there were even brighter spots of warmth, spots that moved about on glowing feet and sniffed blindly at the air with tiny glowing noses. Rats.
On and on moved the rat, its little pinkish feet pattering almost silently on the oxidized metal surface of the rails. 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 rodent teeth in a yellow-daggered snarl of hate.
The rat that was not a rat became suddenly motionless, its sharp little nose pointed directly at the oncoming 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 backward faster than any real rat could have moved.