"

"How do you get your carrier wave underground to those tunnels?" Stanton
asked. "And how do you keep the Nipe from picking up the radiation?"

The colonel grinned widely. "One of the boys dreamed up a real cute
gimmick. Those old steel rails themselves act as antennas 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, but it's running smoothly now."

Stanton rubbed his head thoughtfully. _Damn these gaps in my memory!_ he
thought. It was sometimes embarrassing to ask questions that any
schoolboy should know the answers to.

"Aren't there ways of detecting objects underwater?" he asked after a
moment.

"Yes," said the colonel, "several of them. But they all require beamed
energy of some kind to be reflected from the object we want to look at,
and we don't dare use anything like that." He sat down on one corner of
the table, his bright blue eyes looking up at Stanton.

"That's been our big problem all along," he said seriously. "We have to
keep the Nipe from knowing he's being watched. In the tunnels
themselves, we've only used equipment that was already there, adding
only what we absolutely had to--small things. A few strands of wire, a
tiny relay, things that can be hidden in out-of-the-way places and can
be made to look as though they were a part of the original old
equipment. After all, he has his own alarm system in that maze of
tunnels, and we have deliberately kept away from his detecting devices.
He knows about the rats and ignores them. They're part of the
environment. But we don't dare use anything that would tip him off to
our knowledge of his whereabouts. One slip like that, and hundreds of
human beings will have died in vain."

"And if he stays down there too long," Stanton said levelly, "millions
more may die."

The colonel's face was grim as he looked directly into Stanton's eyes.
"That's why you have to know your job down to the most minute detail
when the time comes to act. The whole success of the plan will depend on
you and you alone."