How to play Treasure island, the Nurikabe puzzle

The puzzle behind Treasure island: Nurikabe

Nurikabe is a pencil puzzle usually credited to Nikoli, the Japanese puzzle publisher that also gave sudoku its name. It is named after the nurikabe of Japanese folklore, an invisible wall that blocks a traveller's way at night. In the classic puzzle you shade squares to build that wall around numbered islands of unshaded squares.

Also known as Islands in the Stream, Cell Structure.

The rules are the classic ones, told as a seaside story: the shaded wall is sea, the unshaded squares are sand, and every number is a treasure chest. The ban on 2 by 2 blocks of sea keeps the sharks away. You finish by flooding every sea square, so sand dots are optional. Maps grow from 5 by 5 to 10 by 12, and a sea square placed on sand costs one of three hearts.

How to play Treasure island

Pebble the otter dives for treasure. Flood the sea and every chest opens.

• Each number is a chest on an island with exactly that many sand squares, the chest included.
• Every island has exactly one chest.
• Islands never touch side by side. Corners are fine.
• All the sea is one piece: you can swim from any sea square to any other.
• The sea never fills a 2 by 2 block. That much water is big enough for sharks, and we don't like sharks.
• Flood every sea square to finish. Sand squares can stay blank.
• A sea square on sand costs a heart. You have three. In relaxed mode there are no hearts, and rule breaks show up instead.

What you can never do

• A 2 by 2 block of sea. Sharks could live there, and we don't like sharks.
• Two islands side by side. Corners may touch.

Controls with Pebble the sea otter

• Tap a square to flood it with sea. Tap again to clear it.
• Drag across squares to flood a whole stretch at once.
• Double tap or long press to drop a sand dot, or pick the Sand tool so every tap drops sand.
• With a keyboard, use the arrow keys to move, Space for sea, Enter for sand and Backspace to clear.
• Undo takes back your last move.

Strategies for beginners

Ring finished islands with sea

A 1 is a whole island on its own, so the squares beside it are sea. The same goes for any island that has reached its size: flood all around it at once. On most maps this is where to begin.

Split chests that sit close together

A square that touches two different islands side by side must be sea, or the two islands would join. Look between chests that are two squares apart, in a straight line or diagonally.

Flood what no chest can reach

Count outwards from each chest: a 3 reaches at most two squares away, a 4 three squares. Any square too far from every chest, or reachable only by squeezing past another island, can never be sand, so it is sea.

Keep the sharks out

When three squares of a 2 by 2 block are sea, the fourth must be sand, because four sea squares would make a pool big enough for sharks. That sand then has to belong to an island, which often tells you which chest reaches it.

Keep the sea in one piece

All the sea joins up. If a stretch of sea is boxed in with only one way out, the sea must continue through that gap. And a square whose sand would cut the sea in two must be sea.

Grow islands that have one way out

An island that still needs squares and has only one undecided neighbour must grow there. On harder maps, every way to finish an island may pass through the same square: that square is sand, whichever shape the island ends up with.

Try a what-if on the hardest maps

Every Expert and Master map needs at least one what-if. Imagine a square is sand, follow the simple rules, and see whether one breaks. If it does, the square is sea. This is careful logic, not guessing, and a hint walks you through it.

Quick tips from the game

• A 1 is a whole island already, so every square beside it is sea.
• A square beside two different islands is always sea.
• Squares no chest can reach are sea.
• When three squares of a 2 by 2 block are sea, the fourth is sand. No room for sharks.
• Saturdays and the hardest levels need a what-if: imagine a square is sand. If a rule breaks, that square is sea.

Words to know

Chest

A number on the map, the clue of classic Nurikabe. It gives the size of its island, chest square included.

Island

A group of sand squares joined side by side. Every island has exactly one chest.

Sea

The shaded squares, called the wall or the stream in classic Nurikabe. All the sea joins up into one piece.

Sand

An unshaded square that belongs to an island. A sand dot is your note that a square is not sea.

Pool

A 2 by 2 block of sea. Classic Nurikabe forbids it; here it is where the sharks would live.

Unreachable square

A square too far from every chest to join any island, so it must be sea.

What-if

Also called a trial: assume a square is sand and look for a broken rule. The hardest maps need it.

Questions about Treasure island