Virtually all water contains particulates . . . if the water is still long enough, those particles precipitate out and form silt. If you disturb that kind of sediment, it comes up into the water, and if it is thick enough, it can literally reduce the visibility to where you can't read your gauges, even if they are pressed to your mask.
I had never experienced true zero viz until last December, when Dive-aholic (who is a friend) told us he took students through a particular passage in one of the Florida caves, to get the experience of zero visibility. He asked us if we wanted to try it, and we said yes -- having never been through this, we felt it was something we should see. Or not see . . . we went through the designated passage, and on our attempt to exit, I could not see the display from my Liquivision, unless it was pressed to my mask. The only information I had on up and down was my exhaust bubbles, and I'm pretty sure that at one point, I swam my head into the sediment. The disorientation and stress of true zero viz has to be experienced to be understood.
I had never experienced true zero viz until last December, when Dive-aholic (who is a friend) told us he took students through a particular passage in one of the Florida caves, to get the experience of zero visibility. He asked us if we wanted to try it, and we said yes -- having never been through this, we felt it was something we should see. Or not see . . . we went through the designated passage, and on our attempt to exit, I could not see the display from my Liquivision, unless it was pressed to my mask. The only information I had on up and down was my exhaust bubbles, and I'm pretty sure that at one point, I swam my head into the sediment. The disorientation and stress of true zero viz has to be experienced to be understood.