"The Secret of Lost Valley" is a short horror story by Robert E. Howard, first appearing in the Spring 1967 edition of Startling Mystery Stories. After the McCrills massacre his kin, John Reynolds, the last surviving member of his family, flees into Lost Valley, seeking refuge. Taking shelter in a cave, John stumbles upon the resting place of a dead McCrill, Saul Fletcher—only to find the body mysteriously vanished. His search leads him to a hidden door and a tunnel that descends into a vast underground chamber. There, he encounters a race of sinister, snake-like humanoids.