Guide lines are absolutely essential in cave diving. At almost any point in a cave dive, there's a risk of disturbing silt and losing visibility. Cave divers ha

Guide lines are absolutely essential in cave diving. At almost any point in a cave dive, there's a risk of disturbing silt and losing visibility. Cave divers have developed a sophisticated set of tools and techniques for navigating in conditions of low or zero visibility. Many underwater cave entrances are marked with a stark warning sign bearing an image of the grim reaper, imploring the diver to "prevent your death - go no farther". Underwater caves can be enticing to divers, but entering one without the correct training and equipment is extraordinarily dangerous. Hundreds of scuba divers have swum into an inviting-looking cave with crystal clear water, then turned around to see a wall of darkness from the silt they have kicked up; very few have lived to tell the tale. https://en.wikipedia.org/wiki/Distance_line https://en.wikipedia.org/wiki/Distance_line http://www.washingtonpost.com/news/morning-mix/wp-content/uploads/sites/21/2016/10/grim-reaper.jpg http://www.washingtonpost.com/news/morning-mix/wp-content/up... https://youtu.be/PVmqK5YZuxM https://youtu.be/PVmqK5YZuxM