Deep in the Romanian forest, a team of scientists stumbles upon the ruins of a 13th-century abbey. On further inspection, they make a startling discover - the abbey is built over the entrance to a giant underground cave system. Local biologists believe the cave could be home to an undiscovered ecosystem. So they hire a group of American cave explorers to help them investigate its depths.
Jack and this brother Tyler are thrill-seeking professional cave explorers who run a team of the top divers in the world. The crack unit, which also includes Charlie and Buchanan immediately begin their exploration. But what they find deep inside the caves is not just a new ecosystem, but an entirely new species altogether.