SIX POINT PERSPECTIVE: This allows an artist to create their work on a sphere.
From an artist that uses six point perspective: "The sixth (South) point is missing from five point perspective drawings. Within five point we get half, or a hemisphere, of the visual world around us. To get the rest of the picture, the the whole picture that is, you must add that 
last vanishing point. You would have to turn around and look at the room BEHIND you to see the rest of the room and to find that last point. If you were in the transparent sphere in St. Peter’s Basilica you would have to copy not only what you see in front of you, but everything behind you as well. A good way to do this on flat paper is to draw the last vanishing point on the back side of the first drawing. Yes, I mean on the back side of your first drawing. The same grid will help you finish the total picture on this back side. When the rest of this picture is drawn you have a 360 degree picture in all directions.
For information on six point perspective is used, check out this site: Termessperes
The cache is an ammo can with tradeable items, and it is located on private property with the owner's permission.