The Transition of Juan Romero
"The Transition of Juan Romero" is a short story by American horror fiction writer H. P. Lovecraft, written on September 16, 1919, and first published in the 1944 Arkham House volume Marginalia.
Plot: 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 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. That morning he and Romero are both found in their bunks, Romero dead. Other miners swear that neither of them left their cabin that night. The chasm has vanished as well.