A dive line is a rope or cord typicaly on a reel that helps a diver navigate his or her way back to his point of origin in a submerged cave or wreck that (s)he may be lost in otherwise. Holding on to this line is a good idea so you do not lose track of it in the case you were to kick up silt or dirt that would not settle for some time thus denying you the ability to see the line. Direction markers can be placed on the line to help orientate the diver so he does not venture further into the structure rather than it's intended purpose of guiding you out. Finding another diver's dive line (to me) is always cool. I like to see where this person has gone. This tends to also ensure a safe path, but you lose the thrill of thinking you are the first to see that certain part of the world.