“What are they after?” Morrison asked.

“God knows.” O’Neill leafed intently through the papers on his clipboard. “We’ll have to analyze all our back-order slips.”

Below them, the autofac exploring crew disappeared behind. The helicopter passed over a deserted stretch of sand and slag on which nothing moved. A grove of scrub-brush appeared and then, far to the right, a series of tiny moving dots.

A procession of automatic ore carts was racing over the bleak slag, a string of rapidly moving metal trucks that followed one another nose to tail. O’Neill turned the helicopter toward them and a few minutes later it hovered above the mine itself.

Masses of squat mining equipment had made their way to the operations. Shafts had been sunk; empty carts waited in patient rows. A steady stream of loaded carts hurled toward the horizon, dribbling ore after them. Activity and the noise of machines hung over the area, an abrupt center of industry in the bleak wastes of slag.

“Here comes that exploring crew,” Morrison observed, peering back the way they had come. “You think maybe they’ll tangle?” He grinned. “No, I guess it’s too much to hope for.”

“It is this time,” O’Neill answered. “They’re looking for different substances, probably. And they’re normally conditioned to ignore each other.”

The first of the exploring bugs reached the line of ore carts. It veered slightly and continued its search; the carts traveled in their inexorable line as if nothing had happened.

Disappointed, Morrison turned away from the window and swore. “No use. It’s like each doesn’t exist for the other.”