Friends Don Shirley and Dave Shaw went diving almost 1,000 feet underwater in Bushman's Cave in South Africa when seeking to recover the remains of Deon Dreyer, who had died in the cave a decade before. More people have walked on the moon than have descended to such depths.

Video footage recovered later revealed that Shaw lost his light at depths and became entangled in the lines he was using to hoist Dreyer's body.

Meanwhile, an equipment failure led to Shirley accidentally receiving too much oxygen, which can have serious or even fatal effects. Then he developed a helium bubble that caused him to lose consciousness and let go of the guideline that told him how to get back out of the caves. He was spinning, disoriented, vomiting, searching for the line in total darkness, and not even knowing which was was up towards the surface.

Eventually Shirley righted himself, but he still had to wait in the water for another 10 hours, slowly ascending, because going up to the surface more quickly would have given him the bends, a condition when divers get "bubbles" of air in their blood from returning to the surface too fast.

However, Shaw's body eventually floated to the surface, attached to Dreyer's.

(A riveting and terrifying audio account of the story is available on from NPR's This American Life.)