Possible Clues:

• It's puzzling to return most of the dust that's fine underground to begin with
• Variation of the Latin square puzzle
• Numbers puzzle
• Digital challenge
• Number-placement game
• Kudos built on university logic puzzle
• Game typically with 81 squares
• Puzzle with a 9 x 9 grid
• Number in wrestling 50% down, starts to kit up for game
• Popular number-logic puzzle