The Green Girl is a science fiction novel that explores the possibility of a vast underwater city hidden beneath the Pacific Ocean. Dr. Hartman, a renowned scientist, discovers a prehistoric city while exploring the depths of the Mediterranean Sea. This discovery leads him to speculate that there might be other cities, even more vast and advanced, submerged miles below the surface of the Pacific. The novel follows a group of explorers as they venture into this uncharted territory, encountering strange creatures, ancient artifacts, and the remnants of a lost civilization.