er to form a perfect sphere.

* * * * *

"Why, the thing's made of copper!" Blaine gasped. "Copper plates. It's
a man-made world; artificial. But where are the inhabitants?"

Antazzo laughed uproariously. "Not man-made, my friend," he corrected,
"but preserved by man for his own salvation. A dying world, it was, and
the cleverest scientists in the universe saved it and themselves from
certain death. What you see is merely a shell of copper, the covering
they constructed to retain an atmosphere and make continuation of life
possible--inside."

"Your people live _inside_ that shell?" Blaine was incredulous.

"What else? We must have air to breathe and warmth for our bodies. How
else could we have retained it?"

It was staggering, this revelation. The young pilot could not conceive
of a completely enclosed world with inhabitants forever shut off from
the light of the sun by day and from the beauties of the heavens by
night. Yet here it was, drawing ever nearer, a colossal monument to the
ingenuity and handiwork of a highly intelligent civilization who had
labored probably for centuries to preserve their kind. A titanic task!
Who could imagine a sphere of metal more than twenty-four hundred miles
in diameter enclosing a world and its peoples? A copper-clad world!

They were coming in close now, and the gravitational pull of the body
made itself felt. Blaine was busy with the controls, sending tremendous
blasts from the forward rocket-tubes to retard their speed for a safe
landing. The incredibly smooth copper surface was just beneath them,
stretching miles away to the horizon in all directions.

The inductor compass was functioning. Evidently Io possessed as strong
a magnetic field as did the inner planets. Antazzo now consulted a
chart which he drew from his pocket, and examined minutely the surface
over which they were speeding. Here and there curious designs were
etched on the copper plates, and it was from these he determined their
course. Obviously there was an entrance to this sealed-in world.

* * * * *

When they had proceeded some two thousand miles in a northeasterly
direction Antazzo gave the order to reduce speed. Off at the horizon
there appeared a bulge in the copper surface, a round protuberance that
resolved itself into a great dome-shaped structure as they drew nearer.
A full two hundred feet it reared itself into th