Nurikabe Puzzle islands, sea and clean deduction
Nurikabe is a Japanese-style logic puzzle built from four rules that interact constantly. Numbered cells grow into white islands of exact sizes. Different islands cannot touch along an edge. Every black Sea cell must join one connected region. The Sea may never contain a solid 2×2 block. The challenge comes from satisfying all four constraints at once rather than treating them as separate checklists.
How to play Nurikabe
Every numbered clue already belongs to an island. If a clue says 4, the final connected white region containing that clue must contain exactly four cells total, counting the numbered cell itself. An island may bend and branch as long as all of its cells connect orthogonally. A connected island may contain only one numbered clue, so two different clues can never merge into a single white region.
All cells that are not part of an island become Sea. The Sea uses edge-to-edge connectivity, not diagonal connectivity: every black cell must be reachable from every other black cell by moving up, down, left or right through black cells. At the same time, four Sea cells may not fill the corners of any 2×2 square. That no-pool rule often forces an Island cell when three cells of a 2×2 block are already known Sea.
Diagonal contact is allowed between islands. This is easy to forget if you come from puzzles where same-color regions cannot touch at all. In Nurikabe, two separate islands may meet at a corner; they simply cannot share an edge. The board therefore asks you to think about local island growth and global Sea connectivity at the same time.
A practical solving sequence
Start with clue sizes you can constrain immediately
A clue of 1 is already complete. Every orthogonal neighbor of that clue must therefore be Sea. Larger clues near borders and corners also have fewer possible growth directions, making them useful anchors. When two numbered clues are separated by a single cell, that middle cell cannot belong to either island without causing two clues to merge, so it must be Sea.
Watch islands before they become too large
When an island component already reaches its clue number, every orthogonally adjacent undecided cell must be Sea. Conversely, if an island still needs cells and only a narrow set of reachable cells can complete its size, those cells become strong candidates for Island. The live island-size counters in the game make this bookkeeping visible without telling you which unknown cell is correct.
Use the 2×2 rule as an active deduction tool
Do not wait until the end to look for illegal pools. Whenever three cells of a 2×2 block are Sea, the fourth must be Island. This creates a local deduction that can propagate into an island-size clue. The game highlights an actual 2×2 Sea violation immediately so you can trace the conflict instead of finishing a large board and discovering the mistake much later.
Protect Sea connectivity
The Sea must eventually form one network. A narrow undecided bridge between two Sea regions can become critical: if turning that bridge into Island would permanently separate the Sea, the bridge must stay Sea. On partially solved boards there may be several temporary Sea segments, so the live monitor describes them as open segments rather than falsely calling them an error while undecided cells can still connect them.
Why this implementation is different
The browser game supports three input styles. Cycle mode is compact for careful solving: tap an undecided cell to move through Unknown, Sea and Island states. Sea Paint and Island Paint are faster on larger boards and support drag input, so a deduction affecting a run of cells does not require repeated mode switching. Desktop players can also right-click a non-clue cell to mark Island directly.
Undo and Redo preserve a substantial move history. Check inspects only the cells you have actually decided and reports conflicts without silently rewriting the board. Verified Hint reveals one reference-correct cell and clearly marks the run Assisted. Pause stops the local timer and hides the board. Seeded Practice and the separate Daily puzzle both autosave in browser-local storage, so leaving the page does not require starting over.
The live rule monitor is intentionally explanatory rather than decorative. It reports how many cells are decided, how many numbered islands have reached their exact size, how many 2×2 Sea pools currently exist, and how many Sea segments your current marks create. A compact counter for every clue shows current connected Island size versus target size. These signals help you understand why a board is becoming impossible without replacing the deduction process.
Verified puzzle provenance
The bundled puzzles do not come from a scraped puzzle pack, a remote API, or a hidden paid service. A project-local generator first constructs legal solved layouts, then places numbered clues. A separate exact-cover-style backtracking solver independently counts legal solutions from the clues alone. A source is accepted into the frozen bank only when that solver finds exactly one solution and that solution matches the generated layout.
The frozen bank contains 60 source boards: 20 Easy 5×5, 20 Standard 7×7 and 20 Challenge 9×9. Each source can be transformed through the eight square symmetries while preserving the puzzle rules. The independent release verifier checks source structure, source uniqueness, transformed validity, thousands of seeded Practice selections and the full 2026 Daily schedule. This is a trust mechanism for the product, not a claim that every Daily date must show a never-before-seen geometric board.
Because the game is local-first, your practice history, clean versus Assisted records, best times and Daily streak live in this browser. There is no global leaderboard, population percentile, IQ label or diagnostic score. The goal is to offer a reliable logic puzzle with transparent local progress rather than attach unsupported meaning to a solve time.
Easy, Standard and Challenge
Easy uses a 5×5 grid so new players can learn the four rules without excessive bookkeeping. Standard moves to 7×7 and creates longer interactions between island growth and Sea connectivity. Challenge uses 9×9, increasing the number of simultaneous regions and the distance over which a connectivity decision can matter. The difficulty labels describe the bundled product modes; they are not population-based ratings.
If you are new to Nurikabe, begin with Easy and deliberately mark both Sea and Island cells instead of trying to hold white-cell deductions in memory. When you can explain why every mark follows from a rule, move to Standard. Challenge becomes much more manageable once you routinely scan completed islands, three-corner 2×2 patterns and potential Sea bottlenecks after every major deduction.
For repeat practice, open a fresh seeded board or put Nurikabe into My Mix alongside other logic games. If you prefer one shared date-stable puzzle, use Daily Nurikabe. The specialist Daily is intentionally separate from the platform’s frozen Today’s 6 rotation, so adding this game does not rewrite existing Daily history.