Pellucidar by Edgar Rice Burroughs [1915]

In this, the first (of many) sequels to At the Earth's Core, David Innes returns to Pellucidar, the world inside the Earth. Burroughs would revisit Pellucidar in later yarns with his consumate hero, Tarzan. But for now, this wraps up the first two Burroughs Hollow Earth novels nicely, as (spoiler alert) Innes saves the inner world and reclaims his lost love, Dian, from his arch-enemy.