ly of
fresh oxygen, but even more, without pumps to move the air in each
compartment they would soon suffocate from the accumulation of carbon
dioxide in the air they breathed out, or bake from the heat their bodies
radiated. On the other hand, the yeasts and algae required carbon
dioxide and yielded copious amounts of oxygen as they grew.

In Roger Hunter's little orbit-ship the ventilation shafts were small, a
loose network of foot-square ducts leading from the central pumps and
air-reconditioning units to every compartment in the ship. But in a ship
of this size....

The grill was over a yard wide, four feet tall. It started about
shoulder height and ran up to the overhead. The ducts would network the
ship, opening into every compartment, and no one would ever open them
unless something went wrong.

And then he was laughing out loud, working the grill out of the slots
that held it to the wall, trying to make his hands work in his
excitement.

He knew he had found his answer.

The grill came loose, lifted down in a piece. He stopped short as
footsteps approached in the corridor, paused, and went on. Then he
peered into the black gaping hole behind the grill. It was big enough
for a man to crawl in. He shinned up into the hole, and pulled the grill
back into its slot behind him.

Somewhere far away he heard a throbbing of giant pumps. There was a rush
of cool fresh air past his cheek, cold when it contacted the sweat
pouring down his forehead. He could not quite stand up, but there was
plenty of room for him to crouch and move.

Ahead of him was a black tunnel, broken only by a patch of light coming
through the grill that opened into the next compartment. He started into
the blackness, his heart racing.

Somewhere in the ship Johnny Coombs and Greg Hunter were
prisoners ... but now, Tom knew, there was a way to escape.

* * * * *

It was a completely different world, a world within a world, a world of
darkness and silence, of a thousand curving and intersecting tunnels,
some large, some small. For hours it seemed to him that he had been
wandering through a tomb, moving through the corridors of a dead ship,
the lone surviving crewman. There was some contact with the other world,
of course, the world of the spaceship outside ... each compartment had
its metal grill, and he passed many of them. But there were like doors
that only he knew existed. He met no one in _these_ corrido