"Good point," said Fred Fine. "Let's steer clear of that. Don't want blasted 'nads."
"I know where it went, but it's not there now," said Virgil. "The rats ate everything. Some rat obviously got a free suprise in with his paraffin, but I don't know where he ended up.' Fred Fine began to point out landmarks: where he had left the corpse of the Microwave Lizard, long since eaten by' you know what; where Steven Wilson had experienced his last and biggest surprise; the tunnel that led to the Sepulchre of Keldor. His voice alternated between the pseudo-scientific dynamo hum of Fred Fine and the guttural baritone of the war hero. We had heard this stuff from him for a couple of weeks now, but down in the tunnels it really started to perturb us. Most people, on listening to a string of nonsense, will tend to doubt their own sanity before they realize that the person who is jabbering at them is really the one with the damaged brain. That night, tramping through offal, attacking giant rats with a strobe light and listening to the bizarre memoirs of Klystron, most of us were independently wondering whether or not we were crazy. So when we asked Fred Fine for explanations, it was not because we wanted to hear more Klystron stories (as he assumed); it was because we wanted to get an idea of what other people were thinking. We were quickly able to realize that the world was indeed okay, that Fred Fine was bonkers and we were fine.
Hundreds of cracked and gnawed bones littered one intersection, and Virgil identified it as where he had discovered the useful properties of the Sceptre. This area was high and dry, as these things went, and many rats lurked about. Virgil switched the Sceptre on for good, forcing them back to the edge of the dark, where they chattered and flashed their red eyes. Hyacinth stuffed wads of cotton in her ears, apparently in case of a shootout.
"Let's set up the 'scope," Virgil suggested. Casimir swung off his pack and withdrew a heavily padded box, from which he took a small portable oscilloscope. This device had a tiny TV screen which would display sound patterns picked up by a shotgun microphone which was also in the pack. As the 'scope warmed up, Casimir plugged the microphone cord into a socket on its front. A thin luminous green line traced across the middle of the screen.
Virgil aimed the mike down the main passageway and turned it on. The line on the screen split into a chaotic tangle of dim green static. Casimir played with various knobs, and quickly the wild flailing of the signal was compressed into a pattern of random vibes scrambling across the screen. "White noise," said Fred Fine. "Static to you laymen."
"Keep an eye on it," said Virgil, and pointed the mike down the smaller side tunnel. The white noise was abruptly replaced by nearly vertical lines marching across the screen. Casimir compressed the signal down again, and we saw that it was nothing more than a single stationary sine wave, slightly unruly but basically stable.
"Very interesting," said Fred Fine.
"What's going on?" Sarah asked.
"This is a continuous ultrasonic tone," said Virgil. "It's like an unceasing dog whistle. It comes from some artificial source down that tunnel. You see, when I point the mike in most directions we get white noise, which is normal. But this is a loud sound at a single pitch. To the rats it would sound like a drawn-out note on an organ. That explains why they cluster in this particular area; it's music to their ears, though it's very simple music. In fact, it's monotonous."
"How did you know to look for this?" asked Sarah.
Virgil shrugged. "It was plausible that an installation as modern and carefully guarded as the one I saw would have some kind of ultrasonic alarm system. It's pretty standard."
"Very interesting," said Fred Fine.
"It's like sonar. Anything that disturbs the echo, within a certain range, sets off the alarm. Here's the question: why don't the rats set it off?"
"Some kind of barrier keeps them away," said Casimir. "I agree. But I didn't see any barrier. When I was here before, they could run right up to the door-- they had to be fought off with machine guns. Thay must have put up a barrier since I was last down here. What that means to us is this: we can go as far as the barrier, whatever it may be, without any fear of setting off the alarm system."