tunnel, dimly
lighted by small globes of violet radiance set at intervals in the
glassy ceiling. After thirty yards of travel along this tunnel they
found their way barred by a massive door of copper-colored metal.

At Layroh's imperious gesture the men halted a dozen feet back of him in
the tunnel while he brought something out of his leather belt-case.
Foster was the only one of the group who was near enough to see that the
object was a small tube closely resembling a pocket flashlight.

The only break in the surface of the great door was a six-inch disk over
near its right-hand edge. Layroh slid this disk aside. Into the opening
that was revealed he sent a series of flashes of colored light from the
tube--two red, three green, and two blue. The colors were the
combination to the light-activated mechanism of the lock. At the last of
the blue flashes there was a whirring of hidden mechanism and the portal
swung slowly and ponderously open.

* * * * *

Layroh beckoned to the men to follow him as he strode swiftly on into a
vast room that was flooded with bluish light from scores of the radiant
globes. As the men passed through the door it reached the limit of its
opening swing and began automatically closing again behind them, but
they were too completely engrossed in the scene before them to notice
it.

They were in a great cavern whose glass-smooth floor was nearly a
hundred yards square, and whose ceiling was so high that it was lost in
the shadows above the maze of metal girders and cables that made a
webwork some forty feet overhead. There was a feeling of almost
incredible age about the place, as though it had been sealed away there
in the heart of the mountain for countless centuries.

On every hand there was evidence that the cavern and all its contents
were the products of a race of beings whose science was one that was
utterly strange to that of the modern world. At the end of the room
where they stood were row after row of different machines, great engines
with bodies of dull silver metal and with stiltlike legs and jointed
arms that made them look like giant metal insects. Foster could
understand few of the details of the machines, but he felt that in
efficiency and versatility they were far ahead of Earth's best modern
efforts.

Grouped together in the center of the cavern were many assemblies of
apparatus linked together by small cables that descended from main
cabl