Surrounded square

"Surrounded square" is an advanced technique used in Nurikabe puzzles. This technique involves identifying squares that are fully surrounded by walls on all four sides (horizontally and vertically).

Since islands cannot be isolated within walls, these surrounded squares cannot belong to an island and must therefore be shaded as part of a wall.

Here's an example of how to apply this technique:

The "Surrounded square" technique is extremely useful in complex Nurikabe puzzles, as it helps to identify certain squares that must be part of the wall.