Pellucidar (Complete) by Edgar Rice Burroughs
Presented in the order in which they were published
Pellucidar is a series of 7 books about a fictional Hollow Earth. 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. Later protagonists include indigenous caveman Tanar and additional visitors from the surface world, notably Tarzan, Jason Gridley, and Frederich Wilhelm Eric von Mendeldorf und von Horst.