Endure - Book 5 (Trapped in the Hollow Earth Novelette Series)

This is book 5.

Three teens are marooned on a what they presume is a mysterious tropical island. Casey and Mike have made their way through the creature-filled jungle to a glass city where they hope to find help for Jack, their abducted friend. Once in the city, they learn shocking news that will change their lives forever, and getting Jack back will be anything but easy.