The Graceful Tree Conjecture is an open problem in mathematics.
Imagine a series of points connected by line segments. This is called a "graph" or "tree." If the points, or vertices, are numbered, the segments can also be numbered by the difference in the numbers assigned to the points (eg, if one vertex is labeled 2 and another is labeled 5, the segment is assigned a value of 5-2 = 3). A tree is considered "graceful" if the vertices can be labeled with numbers (starting from 0), and the segments all end up with different values. A graceful tree is shown in the picture below. The Graceful Tree Conjecture simply states that ALL trees can be labeled gracefully!
This is yet to be proven.
Find this cache on the path from the Bavaria Hills neighborhood. Happy hunting!