It is the frontier of a modern day explorer, a vertical world of slick cliffs and sudden drops and deep in the undiscovered depths of america, called canyoneering, hybrid between mountain climbing and... See More
It is the frontier of a modern day explorer, a vertical world of slick cliffs and sudden drops and deep in the undiscovered depths of america, called canyoneering, hybrid between mountain climbing and cave explorers, places you wouldn't find on any gps. Our team strapped on harnesses, risking life and limb to see never before center rain anse seen terrain and along with them is matt gutman. Reporter: There is a moment when rock walls soaring on either side -- when you realize there is no way out but down. Going back that way is not an option. It's that way or nothing. Reporter: And what follows for a new generation of modern explorers -- the privilege of being the first humans to see this is a gauntlet, frigid water, slippery rappels, skyscraper-high cliffs and this nagging realization that rescue is impossible. Search and rescue guys can't even get to you fast if they knew where you were in the first place. Reporter: No cell phones and in the slot not even satellite phones work. Satellite phones don't even work in the slots. Reporter: You could call them the lewis and clark of canyoneering. You're going to places no one has seen, no human eyes have seen. THAT IS RIGHT. Reporter: You push the edge. Every once in a while you get into a pickle and then you learn something new and try not to make that mistake again. Reporter: It's a sport/activity that includes rappelling, caving and climbing where you knowingly maroon yourself in the bowels of these vertiginous slot canyons. Think of the movie where aron rolston cut his own arm off. Three main ways people can get killed? In a slot canyon, lots of ways to get trapped, hypothermia, get trapped when your ropes don't reach and people die in flash floods. Reporter: The stars of the film "the last of the great unknown" offered to show "nightline" one of their favorite canyons. This is it. Going in hot. Reporter: Canyoneering emerged over the last couple of decades from the domain of tiny sub culture to nearly mainstream. They have explored over 100 un unexplored canyons which they have the privilege of namenaming. Won't be long before we're simg. Reporter: How locold does the water get? The geography is changing around us. This is what it's about, looking at the canyon, so narrow, the colors, the light. This is stunningly beautiful. This is the kind of things that you find when you're willing to go, push it a bit and look in some of these deep, dark corners. Reporter: And we're in the water. Gaspi iing as the cold seizes our bodies. These are called pod holes but they're the size of swimming pools. Some of these can be 20, 30 feet deep. You never commit the entire party to the pool like this or everybody gets trapped. Reporter: You send your most dispensable person -- our senses, the cold, the cathedral canyon walls. Lush, even though this is arizona and then our first rappel. More pod holes and more obstac obstacles. Mora pee rappels. We may be the first to ever see this. The canyon opens grandly into this a bigger canyon, lush with a stream running through it. We break to eat something and marvel at how much is still left unexplored. My guess is there are 300 canyons that are probably worth doing in the grand canyon. And we'll die before we ever see them all. It's just too big. Reporter: Before we know it, thunder starts rolling in, bearing the possibility of flash floods. Almost like clockwork, it's 1:43 and the boomers start it's it's scary because the flash flood comes through there we're in bad shape. And the long, hard slog back up the hill to safety begins. Starting to rain. It's pretty intense lightning. And getting up this hill is hard. What do you think of that, rich? Big boomers. Going to come down. I think I'm glad I'm not in that canyon right now. Reporter: One of the few times he'll ever admit that. I'm matt gutman for "nightline," bear canyon, arizona.
This transcript has been automatically generated and may not be 100% accurate.