Back To The Stone Age by Edgar Rice Burroughs

An American adventurer stranded in the hollow world of Pellucidar is forced from civilization into a brutal struggle for survival among prehistoric tribes and monstrous beasts; stripped of status and repeatedly enslaved, he must rely on cunning, physical courage, and new alliances to endure captivity, loss, and fierce battles, ultimately confronting the world’s savage rules as he fights to reclaim his place and protect those he comes to love.

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