Tarzan: the Lost Adventure is a novel written by Joe R. Lansdale based on an incomplete fragment of a Tarzan novel written by Edgar Rice Burroughs but left unfinished at his death. In Burroughs' last Tarzan story, left unfinished at the time of his death, the ape man plays guardian to an expedition seeking the lost city of Ur. In Lansdale's completed novel, Ur is a society revering a giant and supposedly immortal praying mantis, which is used to slay condemned prisoners in the arena. Tarzan speculates that the creature is originally from the underground world of Pellucidar, to which Ur is connected by a system of caverns and passages. Trapped underground at the end of the story, he seeks escape by seeking out the route to Pellucidar himself.