They came now to one of the walls of the underground cavern, and Old
Beard led them suddenly into a fissure that was well concealed from the
walkways by a tangled screen of vegetation. They stumbled along a narrow
passageway for a few feet, and emerged into a rude shaft, around the
walls of which a roughly-chiseled and steep stairway led upward into
pitch darkness. Here Old Beard halted.

"When I told you there's no way of escape here, it was not that I
haven't tried many times," he said to Dark.

"This shaft leads up into the walls of the structure above--above,
although it is still underground--and I have been up there often at
night. It has long been my hope that I might be able to get a marsuit or
a groundcar and make my escape, but they are kept locked up and always
guarded, against the Jellies and the Toughs.

"I want to take you up and give you an idea of the place now, and later
perhaps you will have some ideas to contribute. Happy and Shadow will
stay down here until we get back."

Old Beard mounted the steep steps slowly, and Dark followed at his
heels. Although the bottom of the "well" was lighted with the same dim
light as that which spread throughout the entire underground area, there
was no light at all higher up, and they had to feel their way carefully
lest they fall off the narrow steps.

At the top, Old Beard stopped and Dark bumped sharply into him.

"I'm going to move down the space between the walls," Old Beard
whispered. "Hold onto my hand and follow me. But don't say anything or
make any more noise than you can help, because anyone beyond the wall
may be able to hear you."

They moved ahead. The way was very narrow, very dark and very difficult,
and frequently was choked with ventilator pipes or tangles of wiring.
They had gone some forty or fifty feet, when Old Beard stopped.

By Old Beard's movements, Dark knew he was working at something. Then a
section of ventilator pipe came away from a ventilator grill, and faint
light illuminated the space in which they crouched. In this dimness, Old
Beard gestured to Dark to look through the ventilator.

Peering out, Dark saw that they were near the ceiling of a large,
high-ceilinged room. In it, under glaring lights, a group of half a
dozen white-clad men were working with knives and other instruments on
the body of a man, either anaesthetized or dead, which lay on a surgical
table.

Old Beard put his face against the grill next to Dark's, and