The Transition of Juan Romero (The Work of H. P. Lovecraft, Episode 7)

The story involves a mine that uncovers a very deep chasm, too deep for any sounding lines to hit bottom. The night after the discovery of the abyss the narrator and one of the mine's workers, a Mexican called Juan Romero, venture together inside the mine, drawn against their will by a mysterious rhythmical throbbing in the ground. Romero reaches the abyss first and is swallowed by it. The narrator peers over the edge, sees something - "but God, I dare not tell you what I saw!" and loses consciousness. The next morning, the narrator and the deceased Romero are both found in their bunks. Other miners swear that neither of them left their cabin that night. The chasm has vanished as well.