Tanar of Pellucidar by Edgar Rice Burroughs

Pirates wreak havoc on Earth's open seas in this classic fantasy adventure by the author of Tarzan of the Apes and John Carter of Mars.
Return once more to Pellucidar, the stone-age kingdom inside the Earth. The dreaded reptilian Mahars have finally been defeated after fifteen years of war. The tribes of Pellucidar have joined together to form an empire ruled by David Innes, one of the explorers from the Earth's surface.
A new threat has emerged: the treacherous Korsar pirates are attacking tribes. To protect the tribes from this new menace, Innes raises an army, but the primitive warriors are no match. Lives are lost, and Tanar of Sair, one of the bravest of the lot, is captured.
The Korsars are desperate to learn the secrets of the empire's weaponry, but Tanar doesn't break. He manages to escape, but when he finally returns, he has terrible news: The pirates have captured Innes!