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The mystical Syvårssøer ("Seven Year Lakes") have their name for the simple reason that they're not always there. From ancient times, people have noticed that the lakes appeared approximately every seven years, or they simply used the magic number 7 for the unexplainable. The fact is that lakes appear and disappear with the height of the groundwater. And even if large amounts of precipitation cause the groundwater level to rise, it doesn't mean that lakes will rise. The groundwater bed has its own "hills and valleys", which is why the Seven Year Lakes are quite unpredictable.