to the opening and saw that his
watch showed one minute of diving time before shortness of breath would
signal time to turn on their air reserve and surface.

Time was critical. If the frogmen didn't go away before their air ran
out, they would have to surface, if they were allowed to by the enemy.
With luck, Scotty could account for one. But that would leave two, both
armed. By this time the first frogman would have blown the water from
his mask and recovered his spear.

No, it would be dangerous for Scotty even to take time for a shot,
unless he could fire without pausing. Their best bet was to make a run
for it, depending on speed.

On land, he was sure he and Scotty could outrun the enemy, but in the
water, speed depended on skill with the fins, and the power of leg
strokes. He doubted that the frogmen were much faster than he and
Scotty, but there was an excellent chance that their speed in the water
was equal.

He conserved his air, spacing his breathing, taking only enough air to
keep comfortable.

There was another scraping sound, and he knew the frogmen were still
around. Were they actually searching the wreck? If so, they might find
the entrance.

And then Rick suddenly discovered a new danger!

Their air bubbles had been floating to the top of the cabin, forming a
pool under the ceiling. But they had stayed in the cabin so long that
enough water had been displaced to bring the pool of exhausted air close
to the entrance, which was only a few inches below roof level.

In a moment the air would spill out, and rising bubbles would warn the
frogmen!

He gripped Scotty's shoulder and pointed to the silvery mass of
exhausted air that curled perilously close to the entrance.

The other boy saw the danger at once. He wrote on his slate, "We go whn
air duz," and held it in the light for Rick to see.

Rick nodded. He drew his belt knife.

There couldn't be many breaths left before the air spilled out. Nor
could there be many before warning constriction forced them to turn on
the reserves. At this depth the reserve wasn't very great.

He saw Scotty reach for his reserve lever and pull it down. A moment
later he had to pull his own.

Something rang like a struck tank, almost directly overhead!

The lip of the bubble pool moved from the water motion caused by pulling
their reserves. Rick watched it, scarcely breathing.

The air pool trembled. A tiny bubble broke loose and sped upward.

Rick squeezed