1st book of the Pellucidar series
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Two explorers burrow deep into the planet's crust and emerge in a savage land inside a hollow Earth, where they have many adventures.
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At the Earth's Core, published in 1914, is the first of a series of science fiction novels written by Edgar Rice Burroughs set inside a hollow earth with a central "sun," a land called Pellucidar. However unlikely this scenario, it allowed Burroughs free play to create heroic adventures in yet another alien environment in addition to his fantastic version of Mars in his Martian series.
Prologue
I: Toward the Eternal Fires
II: A Strange World
III: A Change of Masters
IV: Dian the Beautiful
V: Slaves
VI: The Beginning of Horror
VII: Freedom
VIII: The Mahar Temple
IX: The Face of Death
X: Phutra Again
XI: Four Dead Mahars
XII: Pursuit
XIII: The Sly One
XIV: The Garden of Eden
XV: Back to Earth