Tarzan at the Earth's Core (Edgar Rice Burroughs' Tarzan, #13)

David Innes was a captive in Pellucidar, the strange world within a world that lay under Earth's crust. To rescue him, Tarzan came into that savage, prehistoric land at the head of an expedition equipped with every modern device. But Pellucidar was not like the jungles the ape-man knew. Here were sabre-tooth tigers and every savage creature from the beginning of time. Here the horizon curved back on itself and the sun was always in the middle of the sky. and now, for the first time, Tarzan was hopelessly lost in a land teeming with unknown, gigantic killers...where even time had no meaning! Cover Illustration: Boris Vallejo