arth time, he was circling Titan, while he searched
the grim, forbidden terrain beneath. After days of studying and
speculation he had decided that the Caves must be situated in the
Inferno Range, a place so particularly vicious that no man, so far as
was known, had ever explored it. During the day the heat would boil
eggs, and at night the sub-zero cold cracked great scales off the
granite boulders. And here, too, lay the Trap-Door City of the monster
spiders!

The grim, fantastic range soon appeared over the horizon, stabbing its
saw-tooth peaks far into the sky. Dawn was still lighting the world,
and a great snow-storm, a howling, furious blizzard, concealed the
lower slopes of the mountains. Penrun knew that presently the driving
snow-flakes would change to rain-drops, and the shrieking, moaning
voice of the gale would give way to the crashing, rolling thunder of
the tempest. As the day advanced the storm would die abruptly and the
clouds vanish under the deadly heat.

Then the Trap-Door City, which covered the slopes above the plateau at
the three-thousand-foot level like a checker-board of shimmering,
silken circles, would spring to febrile life as the spider monsters
went streaking and leaping across the barren, distorted granite on the
day's business, the hunt for food in the lowlands, and the opening of
the trap-doors to gather in the heat of the day in the silken tunnel
homes set in the gorges and among the boulders. At sunset the doors
would all be closed, for then the rain and the electrical storm would
return, and at night the blizzard. The storm-and-heat cycle was the
deadly weather routine of the Infernos.

Penrun steered for a tall, cloven peak that towered high above the
Trap-Door City. In its thin air and continuous cold he would be
comparatively safe from marauding spider scouts, and from the peak he
could watch not only the city of the monsters but the better part of
the Inferno Range as well.

He was convinced that before long the mysterious black craft would
put in an appearance somewhere near this spot. Penrun knew it all too
well. There by the cataract of the White River, half a mile across the
plateau from the insect city, he had once been captured.

* * * * *

Next morning when he looked down on the plateau just below the
Trap-Door City he laughed triumphantly. There sat the long
black-hulled space craft he had seen overhauling the liner.

But a moment la