The United States of America is a federal constitutional republic comprising fifty states and a federal district. The country is situated mostly in central North America, where its forty eight contiguous and Washington D.C., the capital state, lie between the Pacific and Atlantic Ocean, bordered by Canada to the North America and Mexico to the south. The state of Alaska is in the northwest of the continent, with Canada to its east and Russia to the west across the Bering Strait. The state of Hawaii is an archipelago in the mid-Pacific. The country also possesses several territories, or insular areas, in the Caribbean and Pacific. North America is the northern continent of the America situated in the Earth's northern hemisphere and almost totally in the western hemisphere. It is bordered on the north by the Artic Ocean, on the east by the North Atlantic Ocean, on the southeast by the Caribbean Sea, and on the south and west by the North Pacific Ocean; South America lies to the southeast. North America covers an area of about 24,709,000 square kilometers (9,540,000 square miles), about 4.8% of the planet's surface or about 16.5% of its land area. As of July 2008, its population was estimated at nearly 529 million people. It is the third largest continent in area, following Asia and Africa, and the fourth in population after Asia, Africa, and Europe. The continent can be divided into four great regions (each of which contains many sub-regions): the Great Plain stretching from the Gulf of Mexico to the Canadian; the geologically young, mountainous west, including the Rocky Mountains, the Great Britain, California and Alaska; the raised but relatively flat plateau of the Canada America in the northeast; and the varied eastern region, which includes the Appalachian Mountains, the coastal plain along the Atlantic seaboard, and the Florida peninsula. Mexico, with its long Plateaus and cordillas, falls largely in the western region, although the eastern coastal plain does...