The Lonely Cache Challenge is a type of Challenge Cache.
The Lonely Cache Challenge involves finding a lonely cache that has been unvisited in over a year. The Wisconsin Geocaching Association host a Lonely Cache Game using Lonely Caches located in Wisconsin.
Usually, there is only a single rule. A cache must be found that has not been found in over a year. Other variations may include how many lonely caches you must find or some kind of point system based on how long a cache went unfound, requiring a certain number of points to claim the challenge.