On any trip underground you're almost guaranteed to ruin one piece of gear or clothing. It's a tough environment, and as such it requires unusually tough gear. The Swaygo is one of the toughest tools I own. The minimalist roll-top bag is made entirely out of polyurethane impregnated and coated nylon, that is tough as nails (and closer to a car tire than anything else I can think of). It has RF welded seams, and the roll-top is locked by a carabiner (unlike a previous bag I used that had plastic clips that failed mid-trip). The roll-top combined with the impregnated nylon makes the bag waterproof, and the toughness of the skin means that even in scrapes and falls the gear inside is kept safe and sound.
Unlike other roll-top bags, the Swaygo is designed with durability, flexibility and tight squeezes in mind. The shoulder straps are made out of webbing, and connect to the bag via three carabiners. The carabiners provide additional utility in that when you are crawling or climbing and need to drag the bag behind you, the top carabiner can be looped and clipped to your leg; when pulled this shortens the shoulder straps, drawing the webbing through a grommet creating a single long leash that minimizes snags during tight crawls.
While not designed for comfort, the bag itself feels great while caving. It's designed to be worn with the rolltop on the bottom, minimizing the bulk at the top of the pack when crawling or duck-walking thereby further reducing snagging while also keeping most of the weight at the bottom of the pack.
Swaygo packs come in three sizes. I own the Push (740 cubic inches) which was perfect for my needs (it swallowed my pocket camera, three extra sources of light, a Nalgene water bottle, granola bars, and extra wool underwear). But for those who need something larger on longer trips, they also make the Pit (950 cubic inches) and the massive Sink (1,200 cubic inches) for $10 and $20 more, respectively.
After a recent caving trip I learned of a cave rescue in Tennessee that was made possible, in part, to four Swaygo packs. By inflating the packs with air and lashing them to the injured caver, the cave-rescue team was able to float the patient out of the cave using their packs as pontoons. I mention this only because in a tough situation I know I can depend on a bag as well-designed and built as the Swaygo. It's built by a caver, for cavers, and as such it has the refined utility that, for me at least, is the definition of a cool tool.
Swaygo Push Pack (740 cubic inches) $79
Pit (950 cubic inches) $89
Sink (1200 cubic inches) $99
Available from and manufactured by Swaygo Gear