of each cell door.

Abbot immediately got busy.

"Can you get word for me at once to Thig?" he whispered to the man on
guard.

"Perhaps," replied that individual non-committally.

"Then tell him," said Abbot, "that I have proof that Dolf is planning
to destroy this city behind him, and never return from the surface."

The sentry became immediately agitated.

"So you know this?" he exclaimed. "How did it leak out? But--through
Milli, of course. And the guard on her cell is not a member of the
expedition! Curses! I must get word to Dolf, and have that guard
changed at once."

And he darted swiftly away.

* * * * *

The young prisoner was plunged into gloom. Now he'd gone and done it!
Why hadn't he first made appropriate inquiries of his guard?

A new guard appeared in front of the door.

"Are you going on the expedition?" asked Abbot.

"Yes, worse luck," replied the guard.

The prisoner forgot his own gloom, in his surprise at the gloominess
of the other.

"Don't you want to go?" he exclaimed incredulously.

"No."

"Why not?"

"Do you know Romehl?" asked the guard.

"Yes," Abbot replied.

"Well, that's why."

"Then you must be Hakin!" exclaimed Abbot, with sudden understanding.

"Yes," replied the other dully.

"You are going on the expedition, and Romehl is not?"

"Quite correct."

"Say, look here!" exclaimed Abbot, and then he launched into the
description of a plan, which just that moment had occurred to him, for
him, Milli, Romehl and Hakin to make their getaway ahead of the
expedition--in fact, that very night--and to set off the time-fuse
before leaving.

It turned out that Hakin knew where the explosives were planted, and
where the submarines were kept, and even how to operate them. He
eagerly accepted the plan; and when next relieved as sentinel, he
hurried away to inform Romehl.

Three hours later he was back on post. Quickly he explained to his
prisoner all about the workings of the submarines of the expedition.
The lights-out bell rang, and all the city became dark, except for dim
lights in the passageways. Hakin at once unlocked the door of Abbot's
cell, and together the two young men sneaked down the corridor to the
cell where Milli was confined.

Silently Hakin and Abbot sprang upon the guard and throttled him; then
released Milli. There was no time for more than a few hurried words of
explanation before the three of them left the prison a