Arto Inkala, a Finnish mathematician, claims to have created the most difficult Sudoku puzzle yet. According to Inkala, it took him three months to finish the construction of the puzzle.
Sudoku is a number placement puzzle. The objective of the player is to fill a 9 x 9 grid with numbers from 1 to 9 so that each column, row, and block (3 x 3 sub grids) contains all the digits. The puzzle was popularized by the Japanese company Nikoli in 1986.
Source: Yahoo News