The sweeping color of sea and sky, blue is a common thread in nature, seen in the cerulean of a whale shark (pictured here), the indigo of a stormy night, and the cobalt of a peacock's feathers. Over the centuries, the hue has come to represent calm, cold, mysticism, and sadness. The Roman aqueduct in Segovia, Spain, is pictured at twilight. Dating to the first century A.D., the well-preserved structure is listed as a UNESCO World Heritage site. A dog jumps into Lake Banyoles in northern Spain. The lake is the country’s second largest.