e's some way," Norman said. "They're slowing--"

The flying-boats were indeed slowing as they dipped lower. They were
very near the dome now, its curving wall a looming, sky-high barrier
before them. Suddenly the boats dipped sharply downward toward the
green sea. Before the two fliers could comprehend their purpose, could
do aught more than draw instinctive great breaths in preparation, the
two craft had shot down into the waters and were arrowing down through
the green depths.

Blinded, flung against his metal strap by the resistance of the waters
they ripped through, Norman yet retained enough of consciousness to
glimpse beams of light that stabbed ahead from the prows of their
rushing boats, to see vaguely strange creatures of the deep blundering
in and out of those beams as the boats hurtled forward. The water that
forced its way between his lips was fresh, he was vaguely aware, and
even as he fought to hold his breath was aware too that the frog-men
seemed in no way incommoded by the sudden transition into the water,
their amphibian nature allowing them to stay under it far longer than
any human could do.

The boats ripped through the waters at terrific speed and in a few
seconds there loomed before them the giant metal wall of the great
dome, going down into the depths here. Norman glimpsed vaguely that
the whole colossal dome rested on a vast pedestal-like mountain of
rock that rose from the sea's floor almost to the surface. Then a
great round opening in the wall; the boats flashed into it and were
hurtling along a water-filled tunnel. Norman felt his lungs near
bursting--when the tunnel turned sharply upward and the boats whizzed
up and abruptly out of the water-tunnel into air!

* * * * *

But it was not the open air again. They were beneath the gigantic
dome! For as Norman and Hackett breathed deep, awe fell on their faces
as they took in the scene. Far overhead stretched the dome's
colossally curving roof, and far out on all sides. It was lit beneath
that roof by a clear light that the two would have sworn was sunlight.
The dome was in effect the roof of a gigantic, illuminated building,
and upon its floor there stretched a mighty city.

The city of the frog-men! Their boats were rising up over it and
Norman and Hackett saw it clear. Square mile upon square mile of
structures stretched beneath the dome, black buildings often of
immense size, varying in shape, but all of