3
8 x x x x x x x x
x x 3 6 x x x x x
x 7 x x 9 x 2 x x
x 5 x x x 7 x x x
x x x x 4 5 7 x x
x x x 1 x x x 3 x
x x 1 x x x x 6 8
x x 8 5 x x x 1 x
x 9 x x x x 4 x x

solver87 originally.
grid437 with naked and hidden singles.
grid157 with naked-singles, naked-pairs, and hidden-singles.

2012, Arto Inkala, Everest, http://aisudoku.com/.
https://sudoku2.com/play-the-hardest-sudoku-in-the-world/

https://sudokuspoiler.com/sudoku/9
8 1 2  7 5 3  6 4 9
9 4 3  6 8 2  1 7 5
6 7 5  4 9 1  2 8 3

1 5 4  2 3 7  8 9 6
3 6 9  8 4 5  7 2 1
2 8 7  1 6 9  5 3 4

5 2 1  9 7 4  3 6 8
4 3 8  5 2 6  9 1 7
7 9 6  3 1 8  4 5 2

8 1 2 ║ 7 5 3 ║ 6 4 9 
9 4 3 ║ 6 8 2 ║ 1 7 5 
6 7 5 ║ 4 9 1 ║ 2 8 3 
══════╬═══════╬══════
1 5 4 ║ 2 3 7 ║ 8 9 6 
3 6 9 ║ 8 4 5 ║ 7 2 1 
2 8 7 ║ 1 6 9 ║ 5 3 4 
══════╬═══════╬══════
5 2 1 ║ 9 7 4 ║ 3 6 8 
4 3 8 ║ 5 2 6 ║ 9 1 7 
7 9 6 ║ 3 1 8 ║ 4 5 2