"What good would that do?" remonstrated the girl. "If you failed to convince Thig, Dolf would make an end of us both. And if you convinced Thig, it would mean the end of Dolf, whose influence is all that keeps me alive. We must think of something else."

"Right, as always," replied Abbot.

A growl came from the doorway. It was Dolf, his bearded face black with wrath.

"So?" he sputtered. "Treachery, eh?"

He whistled twice and two guards appeared.

"Take them to the prison!" he raged, indicating Abbot and Milli. "Our expedition will have to do without a guide. I have learned enough of the American language to make a good start, and I guess I can pick up another guide when we reach the surface." Then, bending close to the frightened girl, he whispered, "And another Empress."

The guards hustled them away and locked them up. As an added precaution, a sentinel was posted in front 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 pa.s.sageways. 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 and made for the locks of the subterranean ca.n.a.l, picking up Romehl at a preappointed spot on the way.

The ca.n.a.l locks were unguarded, as well as the storerooms of the submarines. Each of the rooms held two subs, and could open onto the second lock and be separately flooded.

The submarines were of steel as thick as Abbot's bathysphere. Their shape was that of an elongated rain drop, with fins. In the pointed tip of their tails were motors which could operate at any pressure. At the front end were quartz windows. In the top fin was an expanding device which could be filled with buoyant gas, produced by chemicals, when the craft neared the surface. Each submarine also contained a radio set, so tuned as to be capable of opening and closing the radio-controlled gates of the locks. Each would carry comfortably two or three persons.

Having picked out two submarines and found them to be in order, Hakin sneaked back into the corridor to set off the time-fuse, leaving his three companions in the dark in the storeroom. Abbot put a protecting arm around Milli, while Romehl snuggled close to her other side.

Their hearts were all racing madly with excitement, and this was intensified when they heard Hakin talking with someone just outside their door.

Then Hakin returned unexpectedly.

"Something terrible has happened!" he breathed. "The explosives have been discovered and are gone. One of the expedition men has just informed me. Someone must have gotten word to Thig--"

"Why, _I_ did," interrupted Milli. "I told my guard, just before they came and changed him."

Abbot groaned.

Hakin continued hurriedly: "So Dolf plans to leave at once. He is already rounding up his followers. Come on! We must get out ahead of him!"

An uproar could be heard drawing near in the corridor outside. Abbot opened the door and peered out; then shut it again and whispered, "The two factions are fighting already."

"Then come on!" exclaimed Hakin.

As he spoke he turned on the lights, wedged the door tight against its gaskets and threw the switch which started the water seeping into the storeroom; then he led Romehl hurriedly to one of the two submarines, while George and Milli rushed to the other. Heavy blows sounded against the storeroom door.

The water rapidly rose about them, and the four friends crawled inside the two machines and clamped the lids tight. Then they waited for sufficient depth, so that they could get under way.