At the Earth's Core
The keystone hollow-earth adventure, launching a seven-book series (Tarzan eventually visits). Filmed in 1976 with Peter Cushing and Doug McClure, to the delight of undemanding Saturday afternoons everywhere.
Series Guide
Pellucidar
Burroughs's hollow-earth saga: the savage inner world beneath our feet, lit by an unsetting central sun, where David Innes builds an empire against the reptilian Mahars.