Who am I dealing with?Derec wondered. Newly sobered, he stepped back onto the lift.

“Level Three,” he said.

The next two levels were just as silent and finished-looking as Two. Derec could notdecide whether they were finished-waiting-to-be-used, like the spare parts in the great chamber, or finished-and-abandoned.

But Level Five was another story. The rumble of heavy machinery assaulted his ears even before the platform reached the lighted zone. When he stepped off the lift, he could feel rolling, low-frequency vibrations in the floor and ceiling of the tunnel.

I’m getting closer, he thought. Now-which way? The sound surrounded him, offering no clue as to which of the tunnels had the most promise.

While he stood there equivocating, a double platform arrived and disgorged a porter robot. On impulse, Derec climbed onto its half-full cargo pad. He was counting on its ignoring him, as the picker had. He was not disappointed. Neither cradling him in its arms nor trying to dislodge him, the porter started down the south tunnel.

For the first two minutes of the ride, wind noise and the whine of the robot’s own mechanisms masked the distant work noise. But before long Derec could sort the separate elements: irregular thumping sounds like muffled explosions, a highpitched grinding that made his skin crawl, and a steady background rumble that suggested great masses of rock and ice being moved about.

Presently the end of the tunnel came in sight as a black patch in the distance. Shortly after, Derec began to detect a whiff of ammonia in the air. The moment he did, another piece of the puzzle fell into place.

He had wondered from the start why the complex outside the E-cell was filled with nitrogen. The robots did not require it. Strictly speaking, robots did not require an atmosphere at all. And keeping the complex sealed and pressurized had to be more complicated than simply opening it to space.

But maintaining a standard two-gas atmosphere in the proper proportions through the vast complex was even more complicated. Derec had concluded that the nitrogen atmosphere and “open” breathers were a compromise between the inconvenience of full pressure suits and the complexity of a dual-gas E-system. The nitrogen allowed humans to speak and hear normally and to move about without safesuits, without the fire and explosion risk posed by free oxygen.

But Derec had overlooked something important. The ices which made up a large fraction of the asteroid’s bulk were not water, but compounds like methane and ammonia. The mining processes would inevitably release them as gases into the work area, where they might react with the high energies and circuits of the mining machinery or with each other.

I should have seen it sooner, he thought. Without an atmosphere comprised of some relatively inert gas, there would be no way to dilute the unwelcome compounds or efficiently flush them out. So of course, an atmosphere. Of course, nitrogen. The atmosphere accommodated a human presence, but was not primarily for human convenience.

The porter slowed as they neared the end of the tunnel, and Derec took that opportunity to jump off. Ahead of him were several robots, gathered near the end of the tunnel, and the gateway to what he presumed was the work chamber. Through the gateway he caught glimpses of a ragged rock wall, equipment booms, and an occasional flash of bright light.

The gateway itself was an enormous boxlike machine which filled the tunnel flush to the walls, floor, and ceiling. The only path through to the work chamber was a narrow walkway between columns of bright green chemical storage tanks. That was where he had to go.

As Derec drew closer, he saw that the gateway was actually crawling slowly forward. Like some mechanical larva, the gateway was burrowing through the asteroidal mass and leaving a finished tunnel in its wake. Everything-the raw material of the walls, the covering of reinforcing synthemesh, even the overhead lamps-was being handled in one continuous operation. The gateway was a four-surface paving machine.

But Derec’s real interest was in the excavation beyond. He stepped up onto the gateway and threaded his way between the shoulder-high cylinders, aware as he did that one of the humanoid robots was following him. There was a strong draft through the walkway, from the tunnel to the chamber beyond. Even so, the odor of ammonia was almost strong enough to make him gag.

At the forward end, the narrow walkway widened into a control cabin, where two humanoid robots sat behind a bank of transparent panels looking out into the excavation chamber that surrounded the gateway on three sides. Derec stopped a few steps short of the ramp into the excavation and tried to sort out the functions of the equipment that filled it.

The uncut face of the asteroidal material was some thirty meters away. A two-headed boom cutter was working it, one boom bearing rotary grinders, the other microwave lasers. They moved back and forth like weaving cobras, and the ice and rock wall crumbled before them.

The lasers seemed to be doing most of the damage. Suddenly released from its icy glue, loose rock sloughed off the face with a cracking sound. More resistant deposits were gouged off by the rotating teeth of the grinder. The gases boiling off the face were being sucked into the wide-mouthed exhaust vents that loomed over the work face.

As he studied the work rig, a metallic hand touched his shoulder.

“You may not enter the processing zone during operations,” the robot said.

The robot’s edict stirred a flash of annoyance. “I will if I want to,” Derec snapped back over his shoulder.

The robot tightened its grip pointedly. “You may not enter the processing zone during operations,” the robot repeated. “Untrained personnel are to be considered at risk.”

Shrugging off its touch, Derec turned his back on the robot and looked once more into the excavation. Like the gateway, the mining unit was slowly advancing toward an ever-receding rock face. The motion brought the jumble of loose rock within reach of scuttling scooper arms, which funneled it up a ramp to an enormous hopper. A pair of high-sided conveyors carried material away from the hopper, one to the left and one to the right. While on the conveyor it passed through an N-ray station, an X-ray station, and a magnetometer.

From that point on, things got confusing. It was as though after having gone to all that trouble to mine the asteroid, the robots had forgotten to sort out the part of it they wanted to keep.

Some of the tailings were diverted to a spur conveyor, run through a crusher, and then used as the raw material for the fifteen-centimeter thick walls of the tunnel. To Derec’s astonishment, the rest was carried to the back wall of the work chamber, reunited with the captured methane and ammonia, and built up into a wall of ice and rock again. The excavation never got any larger.

But what about the tunnel? Derec wondered. They have to be taking something out-

Closer study showed him otherwise. The empty volume of the ever-lengthening access tunnel meant only that the asteroidal material surrounding it was being replaced in a more compressed state than it was in when mined. Nothing was being extracted. Nothing was being carried away for later refining or shipment.

It just didn’t make sense.

The depletion alarm on Derec’s first cartridge pack began to sound, and he transferred the delivery tube to the backup. He would have to leave soon or risk dying of nitrogen poisoning before he could return to the E-cell. But it was hard to tear himself away from the incomprehensible sight of a dozen robots and a few million dollars worth of heavy equipment engaged in a task as senseless as trying to dig a hole in water. And how many other excavations just like this were underway elsewhere in the complex? Ten? Fifty? Five hundred?