The new sound was a subterranean one; that is, it seemed to come from far below the house, deep down under the surface of the earth. It might have been earth settling, it might have been a fissure opening and closing, it might readily have been a fugitive temblor, except that it came and went with a certain regularity, as if it were made by some very large thing moving along a colossal cavern far beneath the house. It lasted perhaps half an hour; it seemed to approach from the east and diminish in the same direction in a fairly even progression of sounds. I could not be sure, but I had the uncertain impression that the house trembled faintly under these subterranean sounds.