. >>> serena coffman said a lot of people, when their kids say i want to be an astronaut, she said instead of going up in space to see another world, go diving. that's exactly what will she did down in mexico near teulum, thi is what she saw. pay attention to this right here. she said a lot of divers don't go down through here because it's difficult. one of the things you see is that nylon line. you got to hold on to that. if you don't, it could be a fatal accident. you keep hold of that so you know where to keep going. keep going and if you don't get too nervous, look at the beautiful places you can behold. >> this is where you can get excited. the reason most divers don't do it, one, you need the skill level and it's dangerous. >> she went with a group cave gravity, and they take people through here. wow. just wow. >> now entrances to the cave system that people can get in to go explore. >> look at that open up, though. >> uh-huh. >> an opening to the outside like that. >> they said it's so beautiful. you really have to be very brave, very nonclaustrophobic. >> there are a not of men aro