Though the South Pacific's Esa-ala Caves are some of the least-accessible on Earth, expert diver Frank McGuire (Richard Roxburgh) has explored them for months. On one such expedition, Frank is joined by his teenage son, Josh (Rhys Wakefield), and financier Carl Hurley (Ioan Gruffudd). When a flash flood cuts off their exit, they are caught in a life-or-death situation. With supplies dwindling, the divers must navigate a treacherous labyrinth to find a new escape route or die in the process.
Refusing to believe her story about cave-dwelling monsters, the sole survivor of a spelunking exploration gone horribly wrong is forced to follow the authorities back into the caves where something awaits.
A year after a severe emotional trauma, Sarah (Shauna Macdonald) goes to North Carolina to spend some time exploring caves with her friends; after descending underground, the women find strange cave paintings and evidence of an earlier expedition, then learn they are not alone: Underground predators inhabit the crevasses, and they have a taste for human flesh.