hey _must_ play
baseball and they _must_ swim.

Bubbles went overboard at about three o'clock. There were twenty or
thirty boys of all sizes already in the water, and the addition of one
to the struggling group of wet heads was not to be noticed. Nor was the
disappearance of that head noticed, nor the fact that it appeared to
remain under water for nearly three-quarters of an hour, nor that when
it finally did emerge it looked on the whole as if it had seen a ghost.

Bubbles, it seems, was less interested in the waters around Pier 31A
than in the waters underneath. And for this reason: on the previous
night, while stripping for a swim, he had heard a muffled sound of
voices coming from directly under the pier, followed by a long subdued
roaring as of a load of earth being emptied into the water. Now, under
Harry West's tuition Bubbles had formed the habit of investigating
whatever he did not understand. And he wished very much to find out why
people should talk under piers at night, how they could get under Pier
31A except by swimming, and _if_ they were throwing earth overboard
_why_ they were doing so, and where they got the earth.

His head filled with vague and highly colored notions of a smugglers'
cave, his narrow lungs filled with air, Bubbles dove, swam between two
slimy barnacled piles, and came up presently in a dark, dank, stale,
gurgling region, wonderfully cool after the blazing sunlight which he
had just left.

Toward the shore the light that filtered between the supporting piles of
Pier 31A became less and less, until completely shut off by walls of
solid masonry. Into this darkness Bubbles swam with great caution,
accustoming his eyes to the obscurity and holding himself ready to dive
in retreat at the first alarm.

The shore end of Pier 31A had originally been a clean wall of solid
masonry. The removal of half a dozen great blocks of stone had made a
jagged opening in the midst of this, and into this opening, pulling
himself a little out of the water, Bubbles strained and strained his
eyes and saw nothing but the beginning of a passageway and then
pitch darkness.

His heart beat very hard and fast like the heart of a caught bird. Here,
leading into the city from the shore of the East River, was a mysterious
passageway. Who had made it and why? There were two ways of finding out.
One was to wait patiently until some one entered the passage or emerged
from it. The other way, and the better, was to