Hidden in the tree-covered mountains a few hours from Tokyo lies a simple puddle with a big secret. It never dries up, and attracts wildlife including green pigeons who gulp down its mysterious waters. The puddle is fed by a natural spring full of minerals including sodium, which the birds crave as a supplement to their fruit-rich diet. But this craving can be their undoing, as keener eyes survey the scene from above. The mountain hawk-eagle can swoop down in an instant, catching prey unawares.