Thunder Under the Badlands: Thunderstruck Series

This science-fiction novel, the first volume of the Thunderstruck Series, describes the discovery of a prehistoric world that was created by the draining of the Western Interior Cretaceous Seaway. It exists in what could be described as a massive geode that is located beneath the Badlands of South Dakota, the formation of which resulted from the whirlpool that took the inland ocean underground. The ecosystem of this environment is sustained by chemosynthesis, the same process supporting life around the hydrothermal vents at the bottom of the world's oceans. What was supposed to have been an unmanned incursion becomes first a rescue mission and then a fight for survival when the top-secret rescue vehicle is badly damaged. The unwitting explorers find themselves at risk from not only a host of prehistoric creatures but also possible exposure to a lethal microorganism that is found to inhabit this domain. Unfortunately, even these terrors pale in comparison to the nightmare they discover living in the one section of the immense cavern they had believed to be safe, a predator more horrifying than anything that has walked the face of the earth since before the age of the dinosaurs. And it has just found its way back to the surface!