sentences:  In cave systems with only one opening, line arrows will always indicate the route back to the entry, but in cave systems with multiple openings, they may not point to the entry used by the dive team, and in some cases this may not be a usable exit.Some popular cave diving systems have permanent line arrows at regular intervals pointing to the nearest exit, and may be marked with a distance to the exit.Permanent arrows may also be used to identify secondary passages and midpoints in a cave.