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.