take some
disturbance that could reach down a hundred and twenty feet to break off
a chunk."

Rick grinned. "You're right. But we haven't anything to lose by taking a
look, have we?"

They trotted down the beach toward their own house at a half run. Rick
looked at his watch. "At least one pair of tanks should be full by now,
and there's plenty of time for a dive. Come on!"

They paused at the pier, put the pressure gauge on the first two tanks
in series, and found them charged, as Rick had predicted. Then they ran
for the house.

Zircon and Tony were gone and there was a note on the living-room table.
"_We're exploring the southern end. Be back in an hour or two._"

"Shall we wait?" Scotty asked.

"No need. We can take our floats. Let's get going."

They changed to trunks. Then, since they would not have anyone on the
surface to keep track of time or depth, strapped on wrist watches,
compasses, and wrist depth gauges. Floats and weight belts were put on,
then the boys added small plastic slates and pencils for writing
underwater. Knives, masks, snorkels, their favorite guns, fins, and
lungs completed their equipment.

"Shall we walk up the beach, or swim?"

"Swim," Rick said promptly. "This stuff is too heavy to carry
comfortably."

They launched floats, placed aqualung mouthpieces on top of their masks,
and swam parallel to the beach. By using snorkels they avoided the
effort of lifting their faces out of water to breathe and conserved the
air in the tanks. With effective but effortless leg strokes they moved
along rapidly.

As they approached the ship rib that Scotty had found they turned and
swam straight out toward the reef, crossed it, then came to a halt.

"Let's tie our floats to something," Rick suggested, and Scotty nodded.

Aqualung mouthpieces replaced the snorkels, and each boy tested his flow
of air, checked to be sure his mask was connected to the lung by a
safety line, charged his gun, and set his watch. The watches, designed
especially for underwater swimming, had an outer dial that could be set
to show elapsed diving time.

Rick hooted and pointed down. Scotty nodded and they submerged. Because
of their belt weights, and the weight of air in their tanks, they were
just heavy enough to sink slowly. After the dive, when the air in the
tanks was nearly exhausted, they would weigh about five pounds less and
have a slight positive buoyancy that would help them to rise.

They found coral