At the Earth's Core by Edgar Rice Burroughs

Dr. Abner Perry has invented a high-calibration digging machine affectionately called 'The Iron Mole'. While testing his invention with his financial backer and former student David Innes, the machine malfunctions and the pair end up burrowing deep into the earth to emerge in Pellucidar, a lush underground cavern filled with giant prehistoric creatures. While fleeing one such creature, Dr. Perry and David are captured by strange inhuman soldiers, called Sagoths, and placed with other human slaves, where they meet Ghak and the beautiful Princess Dia. Dia is kidnapped by another human named Hoojah the Sly One, while Dr. Perry, David and the slaves are taken to the city of the Majars, large telepathic bird-like creatures that rule the underground world. While David is sent to repair the walls that protect the city from the molten lava, Dr. Perry is sent to transcribe books in the Majar's library. David is able to escape his captors and finds a secret passage out of the Majar city. Outside, David meets Ra, the chief of a human tribe. David suggests that Ra organize the tribes to defeat the Majar but Ra shows David the Majar's true power by taking him to the Majar's grotto where he witnesses one of the Majars hypnotize a female slave before swooping down and carrying her off in its powerful talons. While sneaking back into the city, David and Ra are captured and forced to battle a huge monster but they prevail, killing a Majar in the process. Seeing that the Majar are not invincible, the slaves revolt, allowing David and Ra to escape with Ghak and Dr. Perry. Along the way, Dr. Perry shows David the 'secret of the Majar', a nursery where all the Majar are born. David vows to destroy the Majars but first, he must rescue Dia from Jubal the Ugly One. With the aid of Ra and Ghak, David unites the human tribes and arms them with primitive weapons but the telepathic Majar are prepared for their attack. At first, the battle doesn't go well, with Dia and Dr. Perry being captured but Ra is able to destroy the nursery by unleashing the lava at the cost of his own life. Hypnotized by a Majar, Dia is about to be killed when David and the other humans arrive to save her and Dr. Perry. As the humans flee the city, it is consumed by lava, killing all the Majar. Returning to the surface, David asks Dia to come with him but she says she cannot and the two sadly part company.