Click a cell to shade it as sea; that's all you need to solve it. Click again to dot it as a confirmed island, just to help track your own deductions. Click once more to clear.
A solved 5×5 example. Each number is one island of exactly that many white cells; every dark cell is sea, and all the sea forms one connected shape.
Tips: a “1” means every cell touching it is sea, so start there. And since no 2×2 patch of sea is allowed, a cell boxed in by sea on two sides is often forced to be white.
Nurikabe (塗り壁, “plastered wall”) is named after a yōkai from Japanese folklore: an invisible wall said to block travelers on lonely roads at night. It can’t be climbed or walked around; some tellings say the only way past is to strike low with a stick.
The puzzle itself was introduced by Nikoli, the Japanese publisher behind many of the world’s best-known logic puzzles (Sudoku among them), in the early 1990s. Solve one, and the shaded cells trace out exactly that kind of wall: a single connected shape hemming in every island.