Cave and Wreck Penetration Divers from all around the world use Line Arrows to Mark the Direction of the Closest-Exit. Line Arrows are used by cave and wreck divers to point in the direction to go. Line Arrow attaches to the safety line placed through the cave tunnels or passage ways of a wreck keeping the divers safe even if they disturb the sediments and visibility is lost. Line Arrows are made from Durable Strong Non-Corrosive Plastic Material for years of Safe-Diving. The Aqua Line Arrows are available in several colors and a Glow-in-the-Dark" version for Low-Light Conditions.