Late on a sweltering morning in April, two slim British cavers named Frank and Cookie lowered themselves into a slick, humid pit deep below Borneo's rainforest.
Connecting the two would create a "super system," one of the longest subterranean labyrinths on the planet. As the men wormed down, drilling and hammering bolts into the slick rock to hold their climbing ropes, their odds of success seemed good.