Instead of following immediately he belly-crawled to the edge of the flat roof. Two armored cars were approaching, still hidden from each other by the curving street, but he could see them both.

Anger at his pursuers burned fiercely inside him, anger and the deep-seated prejudice against purely defensive action that was a legacy from the Special Corps days on Earth.

Smiling grimly, he unslung the rifle he had taken from the girl and sent a single bullet ricocheting harmlessly off the turret of each car.

Then he followed Susan. Even through the massive stone walls of the building they could hear the whistling roar of two proton cannon—firing at each other. Colonel Hammer would be displeased with the survivors, Nick reflected with grim amusement.

They paused just inside the black hole to let their labored breathing return to normal. It seemed to go right through the building, between inner and outer shells.

"We'd better climb down and hope it goes deep enough," he said at last. The Martian Exploitation Company had a little gadget, outgrowth of the last War on Earth, which could detect the presence of living creatures through a hundred feet of solid rock.

"This passage will join the tunnels," the girl said with quiet confidence. "We can dodge their detectors."

"What tunnels? You been here?" he asked sharply, trying to see her face in the blackness.

"No, but a vora made this."

Nick didn't understand, but there was no time for hesitation. They climbed down, into an underworld of blackness and silence. He went first, searching out niches in the almost vertical shaft with his toes, lowering his body, reaching overhead to guide Susan's feet. Once one of them dislodged a sliver of rock that bounced and clicked into the depths for what seemed like minutes. His mind was seething with questions but the treacherous shaft required his full attention.

Only the light gravity of Mars made the climb possible, and even then his muscles were stiff and aching when at last his feet touched a solid floor and they sprawled in what the echoes of their heavy breathing told them was a roughly horizontal tunnel. He estimated they had come at least a mile straight downward, perhaps more.

For a long time they lay without moving in the powder-fine sand that 

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