Horror about a team of scientists who stumble upon the ruins of a 13th century abbey deep in the Romanian forest. On further inspection, they make a startling discovery: the abbey is built over the entrance to a giant underground cave system. The scientists hire a group of American cave-explorers to help them investigate its depths. Jack (Cole Hauser) and his brother Tyler (Eddie Cibrian) are thrill-seeking professional cave explorers who run a team of the top divers in the world. They arrive in Romania with all the latest equipment, including a new type of scuba tank allowing a diver to remain submerged for up to 24 hours. The crack unit, which also includes Charlie (Piper Perabo) and Buchanan (Morris Chestnut), immediately begins their exploration. But what they find deep inside the cave is not just a new eco-system, but an entirely new species altogether with a bloodthirsty aim never to allow the team ever to return to the surface.