lids tight. Then they waited
for sufficient depth, so that they could get under way.

The water rose above their bow windows, but suddenly and inexplicably
it began to subside again. A man waded by around the bow of Abbot's
machine.

"They've crashed in the door, and are pumping out the water again!"
exclaimed Abbot. "We're trapped!"

"Not yet!" grimly replied the girl at his side. "Can you work the
radio door controls?"

"Yes."

"Then quick! Open the doors into the lock!"

He pressed a button. Ahead of them two gates swung inward, followed by
a deluge of water.

"Come on!" spoke the girl. "Full speed ahead, before the water gets
too low."

Abbot did so. Out into the lock they sped, in the face of the surging
current. Then Abbot pushed another button to close the gates behind
them. But the water continued to fall, and they grounded before they
reached the end of the lock. Quite evidently the rush of the current
had kept the doors from closing behind them. The city was being
flooded through the broken door of the storeroom.

But Abbot opened the next gate, and again they breasted the incoming
torrent. This time, although the level continued to fall, their craft
did not quite ground.

"They must have got the gates shut behind us at last," said he, as he
opened the next set and pressed on.

* * * * *

And then he had an idea. Why not omit to close any further gates
behind him? As a result, the sea pressure would eventually break down
the inmost barriers, and destroy the city as effectively as Dolf's
bomb would have done. But he said nothing to Milli of this plan: she
might wish to save her people.

Gate after gate they passed. This was too simple. A few more locks and
they would be out in open water. The submarine of Hakin and Romehl
swept by--evidently to let George and Milli know their presence--and
then dropped behind again. But was it their two friends after all? It
might have been some enemy! They could not be sure.

This uncertainty cast a chill of apprehension over them, which was
immediately heightened by the sudden extinguishing of the overhead
lights of the tunnel. Abbot pressed the radio button for the next set
of locks, but they did not budge.

"What can be the matter?" he asked frantically.

"My people must have turned off the electric current," Milli replied.
"The gates won't open without electricity to feed the motors. We're
trapped again."

For a momen