rom's exploring spear assured them that it was deep water.

"We must swim," said he. "Leave your clubs behind." And leading the
way down into the unknown tide, he struck out straight ahead.

It was nerve-testing work swimming thus through that unseen water to
an unguessed goal; but Grom was unhesitating, and his companions
rested upon his steady will. The water was of a summer warmth, and
slightly salt, which convinced him that it had free communication with
the sunlit tides outside. Several times he came within touch of the
rocky walls of the cavern, and found that they went straight down to a
depth he could not guess. But he kept on with hope and confidence at a
leisurely pace, which, in that bland and windless flood, he knew that
every member of his party could have maintained for half a day.

Suddenly there appeared ahead of them a faint, bluish gleam upon the
water's surface. It was something elusive and unreal, and vaguely
menacing.

"Daylight!" exclaimed young Mo eagerly. But Grom said nothing. He did
not think it was daylight, and he was apprehensive of some new peril.

The strange light grew and spread. It was evident now that it rose
from the water, and also that it was advancing rapidly to meet the
astonished swimmers. After a few moments it was bright enough in its
blue pallor to show the swimmers that they were traversing a vast hall
of waters, whose roof was lost in darkness. Some fifty yards ahead of
them, and a little to the right, a low spit of rock, half awash for
the greater part of its length, ran out slantingly from the wall of
the stupendous chamber.

Toward this ledge Grom now led the way, hurling himself through the
water on his side at top speed. He could not fathom this mysterious
phosphorescence, and he wished to get his people out upon dry land
before it reached them. But fast as the adventurers swam, the ghostly
radiance spread faster. Before they got to the ledge, the light was
all about them; but it seemed to be coming from a great depth.

Nervously they all glanced down, and a low cry of horror broke from
their lips. The depths were swarming with monstrous, luminous forms, a
moon-bright, crawling, sliding field of claws and feelers, and broad,
flat backs, and dreadful, protruding eyes.

The eyes all stared straight up at them with a fixed malignancy that
froze even Grom's blood. They seemed innumerable, and all together
they came suddenly floating upwards.

Already the fugitives