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.
The Tarzan stories represent some of Burroughs' best work. The Pellucidar stories do not. Burroughs stretches credulity in all his stories, but he takes it to the limit in the Pellucidar stories. In the Pellucidar seriest Burroughs employs a preposterous concept (a hollow Earth with an inner world where time stands still) and adds insult to injury with highly improbable plot twists. This makes the quality of "Tarzan at the Earth's Core" all the more surprising. It stands as the absolute best Pellucidar story and one of the best Tarzan stories. Ironically it stands near the middle of both series.
David Innes, the hero of the Pellucidar stories, is in trouble. Jason Gridley, inventor of the Gridley Wave, hears the radio distress signal from the center of the Earth, and organizes a rescue party. Many stalwart adventurers, including Tarzan of the Apes, enlist in the expedition. Where Innes got to the Earth's core in a mechanical mole, Gridley's party travels there in an airship. Read the book to find out how they fly an airship to the center of the Earth and confront the many perils of the savage world they find.