Diver navigation, termed "underwater navigation" by scuba divers, is a set of techniques—including observing natural features, the use of a compass, and surface observations—that divers use to navigate underwater. Free-divers do not spend enough time underwater for navigation to be important, and surface supplied divers are limited in the distance they can travel by the length of their umbilicals and are usually directed from the surface control point. On those occasions when they need to navigate they can use the same methods used by scuba divers. Although it is considered a basic skill, it is normally only taught to a limited degree as part of basic Open Water certification. Most North American diver training agencies only teach significant elements of underwater navigation as part of the Advanced Open Water Diver certification program. Underwater navigation is usually a core component of most, if not all, advanced recreational diver training. In the PADI Advanced Open Water Diver course, it is one of the two mandatory skills (together with Deep diving) which must be taken alongside three elective skills. Training agencies promote underwater navigation as a skill (despite the fact that it is less popular than other recreational diving specialties) on the basis that it: builds diver confidence saves energy by minimising excess swimming makes dive planning more effective keeps dive buddies together reduces air consumption Underwater compass navigation is a component of the scuba-based underwater sport, underwater orienteering. When it is critical for safety to return to a specific place, a distance line is generally used. This may be laid and left in place for other divers, or recovered on the return leg. Use of distance lines is standard in penetration diving, where the divers cannot ascend directly to the surface at all times, and it is possible to lose track of the route out to . Techniques Underwater navigation in recreational diving is broadly split into two categories. Natural navigation techniques, and orienteering, which is navigation focused upon the use of an underwater magnetic compass. Natural navigation, sometimes known as pilotage, involves orienting by naturally observable phenomena, such as sunlight, water movement, bottom composition (for example, sand ripples run parallel to the direction of the wave front, which tends to run parallel to the shore), bottom contour and noise. Although natural navigation is taught on courses, developing the skills is generally more a matter of experience. Orienteering, or compass navigation, is