The Grey Labyrinth, which is paid for by visitor donations and maintained by a collection of volunteers, is an archive of puzzles from mathematicians, philosophers, and others. The editors challenge visitors to solve one of the rotating unsolved puzzles before the solution is posted online. Recently solved puzzles include a relativity game and exercises in geometry and probability. A survey on each solution page asks visitor to rate the puzzle on a scale of one to five, and results for the puzzle are posted online. Puzzles from previous years, going back to 1996, are saved in the archives, which can be sorted by name, popularity and date. Anyone who agrees to the Registration Agreement Terms can register to join the forum or chat on a private messaging system and also contribute to the list of puzzles.