>> even deep divers in open water, we lay lay line. you can follow it in and out, pitch black, or dark, you have that line. that's your lifeline to traverse back and forth. >> saw somebody check the watch. timing is key. >> timing is, yes, i imagine they're not dealing with decompression in this type of thing, not deep enough, not enough water to cause collection of nitrogen in their system, but i would imagine time getting through certain legs, the amount of gas you have is all important. they keep an eye on that. >> let's look at some of the cliffs they had to encounter. it is sleep, slippery, and dark. >> yes. obviously there's dry areas of the cave, but if you put this underwater, have to navigate this, it is an impossible feat.