THAM LUANG CAVE, Thailand — Just reaching 12 boys and their soccer coach trapped in a cave in northern Thailand required a six-hour underground journey that is grueling and treacherous even for the most experienced cave divers: swimming in pitch blackness and vicious currents, squeezing through two-foot-wide passages and climbing over boulders several stories high.

One veteran diver, a former Thai Navy SEAL, lost consciousness and died early Friday after placing spare tanks along the route. Meanwhile, oxygen is starting to run low in the remote cavern where the children have taken refuge.

Three of those in the cavern are reported by the authorities to be weakening, and despite a round-the-clock pumping operation, the threat remains that monsoon rains could push water levels in their precarious refuge even higher.

The initial euphoria in Thailand and around the world that all 13 people had been found alive has given way four days later to deep anxiety over the challenge of getting them out. The option of waiting months until seasonal floodwaters recede now seems less promising, but the practical problems of ferrying 12 children and one adult safely through a nearly three-mile maze of perils remain daunting, all the more so since none of the children are said to be able to swim, much less use diving gear.