Planusfinitorphobia
Fear of the flat horizon - (example: Out at sea, on a boat, strangely calm waters and no landmarks, just a flat stretch of open area surrounding you and a flat line dividing earth and sky.)
My friend has Planusfinitorphobia, they are terrified of being surrounded by flat landscapes or open oceans, without any defining land features to separate land from sky, it makes them really uneasy.