Deep in the Romanian forest, a team of scientists stumbles upon the ruins of a 13th century abbey 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 (Cole Hauser) and his brother Tyler (Eddie Cibrian) are professional cave explorers who run a team of the top divers in the world, including Buchanan (Morris Chestnut) and Charlie (Piper Perabo, Coyote Ugly). They arrive in Romania with all the latest equipment, including a new type of scuba tank that allows a diver to remain submerged for up to 24 hours, and begin their exploration. But what they find deep inside the caves is not just a new ecosystem, but an entirely new species altogether.