Tanar of Pellucidar by Burroughs, Edgar Rice;

This third installment in the classic Pellucidar series returns to the exotic and savage land at the center of the Earth. Led by the American explorer David Innes, the human communities have finally overthrown Pellucidar's slave masters, the dreaded Mahars. The peace, however, is temporary, and the Pellucidarian Empire is faced with a new menace, the deadly Korsar pirates. In the ensuing battle many warriors are lost and one of the most courageous, Tanar of Sari, is captured.

Tanar's captors take him to the horrifying realm of the Buried People of Amiocap and ultimately to the Korsars' dreaded dungeons. He endures these terrors because he knows he must escape. He must return to the empire at all costs and alert the people of the newly won empire of the tragedy that has befallen them—David Innes has been captured by the Korsars. Paul Cook provides an introduction for this Bison Books edition.