Pellucidar by Edgar Rice Burroughs. Pellucidar is a fictional "Hollow Earth" milieu invented by Edgar Rice Burroughs for a series of action adventure stories. The stories initially involve the adventures of mining heir David Innes and his inventor friend Abner Perry after they use an "iron mole" to burrow 500 miles into the earth's crust. This is the second book in the series, the first book in the series was 'At the Earth's Core'.
Pellucidar by Edgar Rice Burroughs. Free audio book summary by Wikipedia and reading by Ralph Snelson.
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