This advanced, 10- to 15-hour circuit is very demanding: it's a three-hour hike to the Good Luck cave (the cave-system entrance), and then a 800m wade through a river channel to Sarawak Chamber. Moving around inside the cave requires some use of fixed ropes. Discovered in 1981, the 700m-long, 400m-wide and 70m-high chamber has been called the world’s largest enclosed space.

Don’t count on seeing much, though – ordinary lights are no match for the ocean of black emptiness.