happened to Mickens, and—"

I laughed harshly. "Don't tell me you're beginning to believe in this monster of ours?"

"Of course not. Mickens simply had an attack of vertigo and toppled off; Forster's active imagination supplied the monster. But that shelf looks treacherous. I'd just as soon not go up there."

"You're not talking like an Exploratory Wing man, Donaldson. But it's okay with me if you want to wait down here. That cave might be a goldmine of artifacts. We ought at least to have a look."

His hard face dropped within his mask. "No—I couldn't let you go alone. You win," he said. "Let's try the cave."

We began the climb—and it was, I saw, a deadly road. It narrowed dizzyingly—and while the drop was only a hundred feet, which a man could survive if he landed right, spacesuits weren't made to take falls of that sort. And without a suit, a man was instantly dead on this methane-ammonia atmosphere world.

We were about ten feet out on the ledge, I in the lead and Donaldson behind me, when I heard him gasp.

"Great God! There it is!"

I felt him lurch against me in sudden terror, nearly heaving me into the abyss, but somehow I steadied myself, dropped to my knees, hung on. I turned.

He had avoided a fall too. But I saw no monster.

"Where is it?" I asked.

"It came out of the air right next to me—just popped out of the void and vanished again. I saw it, though." His voice was hoarse. "I apologize for everything I've said. The thing is real. If it weren't for your sure footing we'd both have gone the way Mickens did."

He seemed almost hysterical. There was no sign of the monster, but I wasn't going to take any chances out on this ribbon of rock with a hysterical man.

"Let's go back," I said. "We'll try to get to the cave some other time."

"All right," Donaldson said, shaken. We turned and inched our way back along the shelf to safety, and half-ran to the sanctuary of the ship.

But once we were inside and I was thinking clearly again, I began to sprout some suspicions.

I reasoned it out very carefully. Every time Donaldson had gone out 

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