"Tarzan at the Earth's Core" is unique in the Edgar Rice Burroughs ouvre because it is a crossover novel. This was the 13th Tarzan novel and the 4th Pellucidar story and not surprisingly ends up being one of the better offerings in both series. Originally published as a seven-part serial in "The Blue Book Magazine" in 1929-30. The story fits better into the Pellucidar series, where it works mainly as a sequel to "Tanar of Pellucidar," and it is Tarzan fans who would be more lost in this one than readers of the Pellucidar books. The plot is standard fare for a ERB novel, involving a rescue mission, with the key difference between not so much Tarzan's involvement as the idea that the person who needs to be rescued is not a damsel in distress but David Innes, first Emperor of Pellucidar.
Innes is being held in the dungeons of the Korsars, and Jason Gridley (inventor of the Gridley wave that allowed ERB to "receive" the Martian stories from John Carter, which accounts for the other major ERB series) persuades Tarzan to come along fr the fun. Gridley builds a zeppelin and uses it to descend into the land of Pellucidar (do not get me started on the physics involved in a lighter than air ship descending to the Earth's core. Once in Pellucidar Tarzan and Gridley have their separate adventures, and ERB seems to go out of his way to come up with new races of people (e.g., the Horibs) and prehistoric type creatures to beleaguer both of the book's heroes. The romance, of course, happens with Gridley, who meets Jana, the Red Flower of Zoram. Even everybody gets back together and they remember why they came to Pellucidar in the first place.
"Tarzan at the Earth's Core" is a solid ERB pulp fiction yarn all things considered. What makes it work is that Tarzan has some competition for the role of hero in the story. He is more of a major supporting character than the lead, because Gridley is the leader of the expedition and even disadvantaged in the jungles of Pellucidar, where Tarzan finds himself quite at home, even with that weird burning sun in the sky that never sets, manages to hold his own for the most part. Burroughs also includes the set up for the next Pellucidar novel, when Lieutenant Wilhelm Von Horst, the mate of the zeppelin, vanishes. Unfortunately he would have to wait until 1935 to be rescued in "Back to the Stone Age." Meanwhile, Tarzan would go back to his usual run of episodes back in Africa.