diff --git a/views/application.coffee b/views/application.coffee
index 7632e07..7374176 100644
--- a/views/application.coffee
+++ b/views/application.coffee
@@ -76,6 +76,13 @@ class Nikoli.Cell
       new Cell(@x, @y - 1, @game)
     ]
 
+  isPool: ->
+    [
+      new Cell(@x, @y + 1, @game),
+      new Cell(@x + 1, @y, @game),
+      new Cell(@x + 1, @y + 1, @game),
+    ].every (cell) => cell.valid(@value)
+
   valid: (value) ->
     0 <= @x < @game.length && 0 <= @y < @game[@x].length &&
       (!value? || value < 0 && @game[@x][@y] < 0 || value >= 0 && @game[@x][@y] >= 0)
diff --git a/views/nurikabe.coffee b/views/nurikabe.coffee
index c0856a5..257da94 100644
--- a/views/nurikabe.coffee
+++ b/views/nurikabe.coffee
@@ -18,7 +18,9 @@ class Nikoli.Nurikabe extends Nikoli.Game
             black_stream.calculate(cell)
           else if !black_stream.include(cell)
             errors.push {row: i, column: j, message: 'The stream must be continuous'}
-          # TODO check for pools
+
+          if cell.isPool()
+            errors.push {row: i, column: j, message: 'There must be no pools.'}
         else if cell.value > 0
           if white_walls.some((wall) -> wall.include(cell))
             errors.push {row: i, column: j, message: 'Each wall must contain exactly one numbered cell.'}