In cave (and occasionally wreck) diving, line markers are used for orientation. Directional markers (commonly arrows) point the way to an exit (although no permanent guideline must exist); non-directional markers ("cookies") are purely personal markers that mark specific spots, or the direction of one's own exit at line intersections / T's. One important reason to be adequately trained before cave diving is that incorrect marking can confuse and fatally endanger not only oneself, but also other divers.
The line arrow has been invented by Forrest Wilson at the Cave diving NSS workshop, inspired by Sheck Exley and other cave diving pioneers, and later, few hundreds of the hand made were sold through Branford Dive Center in North Florida. Soon they became very popular and today are commonly used by underwater cave explorers.[1]