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.
Lovecraft seems to have disavowed the story early in his writing career. He did not allow it to be published in the small press during his lifetime, and it does not appear on most lists of his stories. He seems not to have shown the story to anyone until Robert H. Barlow badgered him into sending him the manuscript so that Barlow could prepare a typescript of it.
Huitzilopochtli, a Mesoamerican deity associated with human sacrifice, is mentioned, indicating that Juan Romero's transition may have been related to a sacrifice ritual.