Carson examined it closely, noticed a depression on its edge which would afford a handhold. His fingers fitted easily into the groove, and he pulled tentatively. The stone moved a trifle and stopped. He pulled harder, and with a sprinkling of dry earth the slab swung away from the wall as though on hinges. A black rectangle, shoulder-high, gaped in the wall. From its depths a musty, unpleasant stench of dead air welled out, and involuntarily Carson retreated a step. Suddenly he remembered the monstrous tales of Abbie Prinn and the hideous secrets she was supposed to have kept hidden in her house. Had he stumbled upon some hidden retreat of the long-dead witch? Before entering the dark gap he took the precaution of obtaining a flashlight from upstairs. Then he cautiously bent his head and stepped into the narrow, evil-smelling passage, sending the flashlight’s beam probing out before him. He was in a narrow tunnel, scarcely higher than his head, and walled and paved with stone slabs. It ran straight ahead for perhaps fifteen feet, and then broadened out into a roomy chamber. As Carson stepped into the underground room—no doubt a hidden retreat of Abbie Prinn’s, a hiding-place, he thought, which nevertheless could not save her on the day the fright-crazed mob had come raging along Derby Street—he caught his breath in a gasp of amazement. The room was fantastic, astonishing. It was the floor which held Carson’s gaze. The dull gray of the circular wall gave place here to a mosaic of varicolored stone, in which blues and greens and purples predominated—indeed, there were none of the warmer colors. There must have been thousands of bits of colored stone making up that pattern, for none was larger than a walnut. And the mosaic seemed to follow some definite pattern, unfamiliar to Carson; there were curves of purple and violet mingled with angled lines of green and blue, intertwining in fantastic arabesques. There were circles, triangles, a pentagram, and other, less familiar, figures. Most of the lines and figures radiated from a definite point: the center of the chamber, where there was a circular disk of dead black stone perhaps two feet in diameter. It was very silent. The sounds of the cars that occasionally went past overhead in Derby Street could not be heard. In a shallow alcove in the wall Carson caught a glimpse of markings on the walls, and he moved slowly in that direction, the beam of his light traveling up and down the walls of the niche. The marks, whatever they were, had been daubed upon the stone long ago, for what was left of the cryptic symbols was indecipherable. Carson saw several partly-effaced hieroglyphics which reminded him of Arabic, but he could not be sure. On the floor of the alcove was a corroded metal disk about eight feet in diameter, and Carson received the distinct impression that it was movable. But there seemed no way to lift it.