Nantucket is an island 30 miles (48.3 km) south of Cape Cod, Massachusetts, in the United States. Together with the small islands of Tuckernuck and Muskeget, it constitutes the town of Nantucket, Massachusetts, and the coterminous Nantucket County, which are consolidated. Part of the town is designated the Nantucket CDP, or census designated place. The region of Surfside on Nantucket is the southernmost settlement in Massachusetts. Nantucket is a tourist destination and summer colony. The population of the island soars from approximately 10,000 to 50,000 during the summer months, due to tourists and summer residents. According to Forbes Magazine, in 2006, Nantucket had the highest median property value of any Massachusetts zip code. The Nantucket Historic District, comprising all of Nantucket Island, was added to the National Register of Historic Places on December 13, 1966. In doing so the National Park Service paid particular note to the settlements of Nantucket and Siasconset. The island features one of the highest concentrations of pre-Civil War structures in the United States.