A yard at left of us lifted a four-foot high, gently curved barricade, stretching from wall to wall⁠—and beyond it was blackness; an utter and appalling blackness that seemed to gather itself from infinite depths. The rose-glow in which we stood was cut off by the blackness as though it had substance; it shimmered out to meet it, and was checked as though by a blow; indeed, so strong was the suggestion of sinister, straining force within the rayless opacity that I shrank back, and Marakinoff with me. Not so O’Keefe. Olaf beside him, he strode to the wall and peered over. He beckoned us.

“Flash your pocket-light down there,” he said to me, pointing into the thick darkness below us. The little electric circle quivered down as though afraid, and came to rest upon a surface that resembled nothing so much as clear, black ice. I ran the light across⁠—here and there. The floor of the corridor was of a substance so smooth, so polished, that no man could have walked upon it; it sloped downward at a slowly increasing angle.