Nurikabe
Shade the sea, isolate the islands
Nurikabe is a Japanese shading logic puzzle with elegant rules that produce deeply satisfying deductions. The grid contains numbered cells representing islands. You must shade the remaining cells to create a connected sea (river) while ensuring each island is the correct size and no 2x2 area is entirely shaded. These three constraints interact beautifully, creating chains of logic that make Nurikabe one of the most respected puzzles in the logic puzzle community.
History & Origins
Nurikabe first appeared in 1991 in the Japanese puzzle magazine Nikoli, which is also responsible for popularizing Sudoku. The puzzle is named after the nurikabe, a wall-like yokai (spirit) from Japanese folklore that blocks travelers' paths at night. Nikoli has been the birthplace of many beloved logic puzzles, and Nurikabe remains one of their most acclaimed creations, prized for its elegant rule set and the deep deductions it produces.
How to Play Nurikabe
Islands
Each numbered cell is part of an island. The number tells you the total size of that island (including the numbered cell itself).
Connected Sea
All shaded (sea) cells must form one single connected group. You can reach any sea cell from any other by moving horizontally or vertically through sea cells.
No 2x2 Pools
No 2x2 area of the grid can be entirely shaded. This prevents large featureless blocks of sea.
Island Separation
Different islands cannot touch each other orthogonally. Each island is exactly the size its number indicates.
Every Cell Decided
Every cell must be either island (white) or sea (shaded). There are no ambiguous cells in the solution.
Use Deduction
Combine all three rules to determine which cells must be sea and which must be island through logical reasoning.
Strategy & Solving Tips
Mastering Nurikabe requires understanding how the three core constraints interact. These techniques form the foundation of expert solving.
- Cells numbered "1" are complete islands — all four orthogonal neighbors must be sea
- If two different numbered cells are only 2 apart, the cell between them must be sea (islands can't touch)
- Unreachable cells (too far from any island to be part of one) must be sea
- Avoid creating isolated sea pockets — if shading a cell would disconnect the sea, it must be island
- Watch for the 2x2 rule: if three cells of a 2x2 square are sea, the fourth must be island
- Expand islands from their numbers outward, tracking which cells could belong to each island
Common Mistakes to Avoid
- Forgetting the 2x2 rule — it's easy to accidentally create a 2x2 block of shaded sea cells when focusing on island sizes
- Not checking sea connectivity early enough — an isolated sea pocket means you need to rethink your shading
- Assuming island cells must extend in only one direction from the number — islands can bend around corners
- Overlooking that two different numbered cells close together force sea between them since islands can't touch
Nurikabe FAQ
What does "Nurikabe" mean?
Nurikabe is named after a yokai (spirit) in Japanese folklore — a wall-like creature that blocks travelers' paths. The puzzle name evokes the idea of navigating around obstacles, much like finding paths through the sea around islands.
Why can't there be a 2x2 sea block?
The no-2x2 rule prevents trivially large seas and forces the sea to have interesting, meandering shapes. It's the constraint that makes Nurikabe uniquely challenging compared to other shading puzzles.
Is guessing ever required?
No. Every puzzle has a unique solution reachable through pure logical deduction. If you feel stuck, there's always a deduction available that you haven't spotted yet.
How do I know where an island ends?
An island with number N must contain exactly N cells (including the numbered cell). Once you've placed N cells, everything adjacent to that island must be sea. Track each island's current size as you solve.
Ready to Play Nurikabe?
Dive into Nurikabe — the island-and-sea shading puzzle beloved by logic enthusiasts worldwide. Three simple rules create endlessly varied and deeply satisfying deductive challenges.