Transcript for Dive teams are preparing to re-trace that nearly 3-miles route back to boys

Jamesank you so much. And the specially trained ve teams areparing to retrace the ne three-mile route to the boys. It's a painstaking process, complicated by windingerns, pitch-k conditions, and new rains that could cause water levelsrise yet agare's Matt Gutman with the treacherous work ahead. Reporter: That rescue operation full of pitfalls. Rescue teams like these trudging througmiles of wet rock. They've been drilling and chipping away Boulders from cave's path. Air tanks strapped on their ckhey wade tou water, somly submergholding supplies in the air to keep them dry. The 90ivers in this nvoy, exhausting crucial oxygen supplies. Thosant pumps striking full throttle to drain millions Lons of water.yesterday, we traveled to a nearby cave with a similar structure. Imagine doing this under W. Otsue is, ifhis thing is filled with water, finding the actual hole is nearly impossible. To bring the rng others out, the mtinational crew of rescuers, guided by S and the lion their head lamps. Asy make their whrough a maze of flooded passageways and narrow tunnels, much of it water. Zens of tm members graphinghe rescue for days. Even practicing puttinll ce on boys the same size as those stranded athletes. Over the severaldays, divers teaching the boys towim and ticing going under water. Divers using a budtem to get them out, two divers per boy, a line tethering team together.the idea, reducing the risk of the boy gettlost in the fa flowing, dark waters. The trek as longsix hours to the team and nearly same to out. The risks punctuated by the death Thursday of ai Navy S.E.A.L. Who rut of air in one of the fed caverns. The local Thai governor saying he hopes thesion can be completed in three to four days before the monsoon rai intensifossibly reflooding the caves. Erasing the progress that's allowed them to attempt this rescue. Matt, we castss enough how dangerous this all is. Some of these boys don't know to swim. And they'll have to take part in a complicave dive et out alive. Reporter: That's right. Diveay the journeyleaves them as brthless as if they'veclimbed mterest. And Thay the water at is so murky, you can't see the flight in front of T face. After 16 days of starvation, and this being theirst dive, truly a remarkable feat, the rescue. Tom? Remarkable is right. Thyou, Matt. And the four boys that have been rescued areng treated

This transcript has been automatically generated and may not be 100% accurate.