Steve Backshall pushes deeper into Papua New Guinea's secret world by climbing into a vast and unmapped cave system. This underground world is flooded with raging water, and moving just a few meters is slow, hard work, but it's worth it, as each new step is pure exploration. Together with a team of expert cavers, Steve maps new passages, and discovers alien life - blind crabs and cave crickets with huge antenna. But, miles underground in the darkness, he suddenly falls ill...