The Intra-Earth Chronicles, Book II: The Aswan Device

In The Aswan Device by Kara Jacobson, noble sisters Sasha and Adrianne, along with their companions, embark on a mission to bring civilization to the surface with a powerful nuclear machine, only to face a formidable adversary who seizes it for himself. As the disruption caused by their enemy tears open a portal to a parallel reality, the sisters must confront new threats and challenges, testing the strength of their love and friendship in this fast-paced fantasy adventure.