Well, I don't know if this really qualifys as an IRAP but figured I'd submit it anyway..... Was diving my favorite cave with my buddy AL and decided to go have a look at a place I hadn't been before on the advice of a friend. This was one of those dives where everything seems like it's falling into place, life is good, and the cave just looks sweet as all heck. I'm cruising up the Hill and take the first jump to the left, and then the next immediate jump to the right, since I haven't been here before I leave a spool tied in on the latter jump. I grab my buddy AL and pull him up close as this is "sorta tight" and I don't want to thrash on anything, I come across a T and head to the left leaving a pin to mark my egress, follow the line down 'till it ends and while I'd like to go further I'm thinking AL might get into the silt so I stop and think to myself "dang, I have to start sidemounting " and retrace my steps....I get back to the T and having plenty of gas left, continue on noting my marker showing the way out. Gosh, it's pretty back here and soon enough, I see another T, once again I leave my marker indicating the exit and head left....now this is pretty tunnel, and definitely not as well travelled as some. Once again I hit the EOL but this time it really doesn't go anywhere so I head back. I hit the T check my gas, and heck I'm a little above thirds so I decide to continue on in hope that I can eventually jump back to the Hill. It really starts to tighten up and since I'm within 100psi of thirds I figure WTF and go to turn.....at that point my slap strap breaks and my mask falls off and hits the floor. I really don't comprehend so I reach out and grab it and try to put it back, but of course it won't stay and it's like ohhhh right.. I inflate the wings a little and whip out the BU mask and throw it on, of course the thing fogs every 45 secs or so but it's not so bad....I remember being proud that I didn't silt out the passage but if I hadn't had the BU mask things might have turned out a little bit different...especially since I'd done a couple of "visuals"..... to make a long story short that's why I carry a back up mask, I've gotten a lot of grief over it from some people but today I got to be like UH-HUH