Off to the Sudoku Olympics: New challenges for long-distance puzzlers and clever obstacle runners. Sudoku has become a popular sport, and once you get a taste for it, you crave daily training sessions. There are more than enough here: 500 puzzles in varying difficulty levels from feather-light to devilishly hard that test endurance and stamina, ensuring that even the most seasoned enthusiasts will be out of breath. But anyone who completes this puzzle marathon is guaranteed to become a pro afterward.