Sudoku fans may want to have a look at Saturdays weekend section which features an extreme Sudoku.
This is a cube made up of nine Sudoku puzzles on top of each other where each plane through the cube forms a valid puzzle and even three of the six oblique planes also form valid puzzles.
Deliberately each plane does not contain sufficient clues on its own to provide a unique logically solvable puzzle. You need to work on both horizontal and vertical puzzles together.
Unfortunately because Papacom's program insists that each puzzle be logically solvable it is not possible to load the incomplete puzzles into the program to work on.
See http://www.sudoku.org.uk/images/Dion%20Cube.pdf for details of the puzzle
Bob