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Comment by gaius | Hacker News Reader

From the surface, it's easy to say just follow the guide cables the divers have no doubt installed

Someone asked my cave instructor, if you lose the line how long should you search for it? The rest of your life, he replied.

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Do the guide lines have any indicators that would show which way goes further into the cave and which way heads toward the surface?

Yes, triangular plastic line arrows, you can tell just by feel which way to go if you are totally silted out - provided you can find the line itself

Most popular caves have permanent lines installed with arrows every ~100 ft or so, maintained by the local community. A common convention is that the main / most navigable path through a system uses a slightly thicker, yellow nylon line (the "gold line"), while side passages tend to use thinner, white nylon. Divers are expected to bring their own reels and markers into the cave to make and mark temporary connections, for example, from open water to the gold line, or jumps from a permanent line into side passages. Reels are also useful tools for locating lost guidelines in zero visibility.

There's an overview of various line markers at https://www.tdisdi.com/cave-diving-directional-and-non-direc..., and a great photo of line arrows and a gold line / jump reel connection on Wikimedia Commons: https://commons.wikimedia.org/wiki/File:Line_arrow.jpg

The trick is, the permanent arrows indicate the closest "exit" to the system, so if there's a 1000m cave connecting two freshwater springs, then at the 500m mark you would usually encounter two back-to-back arrows, one pointing to each exit. The trick is, if you haven't personally explored both exits, you can't be certain that they're both navigable. So if you're 750m into that system, lose visibility, and need to get out by feel, you have to find the nearest arrow and then blindly swim in the opposite direction from what it says is the exit. It's... "fun."

Probably a naive question, but why don't they hook people up to guide cables with another line or cable?

Because then you couldn’t pass a tie-off, where the line is fixed to something like a rock. Also, risk of entanglement.

why not use something like in via ferrata sets - a single easy-to-open carabiner with a short cord, attached to you. when you climb a ferrata, you clip&unclip hundreds of times, nothing particularly complicated or taxing, and in water you would need only a basic one with little resistance on the gate opening part.

you could pass along the cable holding the carabiner in one hand, not slowing you down

You could pass a tie-off if you had two carabiners, as you do in via ferrata. Don't know about the entanglement risk though... I'm not a diver but that sounds way less dangerous to me than losing the cable altogether, since worst case you could just release the carabiner

Another thing to consider is this is just a guideline - 2-3mm nylon, fixed in place often just be wrapping around a handy outcropping. If you tug on it with any force it will come loose. And then you and everyone in the cave with you are in a lot of trouble.

Another thing that guide mentions is that if the nylon guideline is cut, it will rapidly snap and retract and spool up (kind of like a fishing line).

Also I read an accident report of some divers who survived a silt out condition, they spent a lot of time carefully figuring out what direction they should follow the rope out to exit the cave in places where it has been wrapped around a stone a few times to anchor it.

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