He took a step forward and his boot slid on the glass-smooth rock. He had to grab at both sides of the gully to keep himself from falling. Hardly any dust down here, he realized. This rille must be brand-new, maybe still active.

"New' and 'active' were relative terms on the Moon, he knew. A new rille might have opened up only a few thousand years ago. Its activity might be a slight sigh of underground gas every century or so.

A cough. In his earphones Doug heard somebody cough. Couldn't be anybody but Bam.

Slowly, moving cautiously along the slippery rock floor of the rille, both hands extended to touch its steep confining walls, Doug made his way forward.

Another cough, followed by a quick, desperate gasping.

"Bam!" he called into his helmet mike. "Bam, where are you?"

No response. Standing stock-still, Doug listened hard. Is he holding his breath? No, it's just so faint I can hardly hear him.

Doug pushed along the slick arroyo and the sound of Gordette's breathing grew louder. It sounded strained, labored, as if the man were in pain.

"I'm coming, Bam," Doug called again. "I'll be with you in a couple of minutes. Hang on."

"Don't…' Gordette's voice was weak. It broke into a gasping cough.

"Save your breath. I'll be there."

"Careful… the ground… gives way…"

Doug scanned the ground before him in the light of his helmet lamp. It looked solid enough, glassy and slick, but solid rock. Yet he knew this volcanic vent might be no sturdier than a soap bubble.

More coughing from Gordette. He must be almost out of air, Doug realized. Got to get to him quickly.

The smooth rock floor ended abruptly, like a shattered pane of glass. Black nothingness yawned in front of Doug.

And clinging to the edge of the break like a ship-wrecked sailor desperately clutching a piece of flotsam, was the space-suited figure of Leroy Gordette.

He had one forearm hooked on the crumbling edge of the precipice, and the gloved fingers of his other hand. Doug could see the top of his helmet.

"Hold on," he said, and immediately felt foolish. What else was Bam trying to do?

"Don't!" the black man warned. "Fuckin' rock breaks. It's as thin as tissue paper. Brittle, too."

Doug lowered himself to his knees, then got down on his belly and wormed his way toward Gordette.

"Got no purchase for my feet," Bam said, panting. "Every time… I try to haul my ass up… fuckin' rock crumbles more."

"How deep is the hole?" Doug asked. "Can you see the bottom?"

Gordette coughed. "Must go… all the way down… to Chicago. No bottom…"

Inching closer to the man, Doug felt the brittle rock beneath him crack, like thin ice.

He stretched out his arm as far as he could. "Can you grab my hand?"

"I'm runnin' out of air," Gordette said, gasping. "Forget it. Get outta here."

"Grab my hand!" Doug insisted.

"Can't."