. >> reporter: british diver jason mallenson volunteered to escort the first boy out of the cave. it would be one boy to one diver. already wearing a wet suit, each boy walked down a steep slope into the water. now it was time for an absolutely critical step. agreed to in advance by the thai government, each boy would be injected with a sedative, ketamine, by anesthesiologist dr. richard harris. the shot not just to relax each boy, it was to knock him out cold. >> i was not fully confident we could get them out alive. we weren't sure how they'd react in the water, whether the drug would be enough. >> reporter: once sedate a face mask strapped tightly on each one, a potential leak fatal. >> when they start moving, arms moving around, the potential for them to kill themselves, the potential for them to kill us. >> reporter: but he's not one for turning back. he entered the cold, murky water with the boy fort daunting first dive stage. remember, 350 meters long, 3 1/2 football fields. after about 20 painstaking, agonizing minutes, mallenson successfully completed the first stage. wai