United States

The **United States of America** (**USA**), also known as the **United States** (**US**) or simply **America**, is a large country in the middle of North America as well as the Alaskan Peninsula, the Hawaiian Islands, and a number of other islands in the Pacific Ocean and Caribbean Sea. It is bordered by Canada to the north and Mexico to the south; the former is the longest undefended border in the world. Founded by mostly English-speaking colonists from England, Holland, and later Germany, the United States expanded from 13 English coastal colonies to 50 states and 14 territories organized in a federal constitutional republic. The United States is among the world's strongest countries, economically, militarily, and financially. However, the lack of investment into human infrastructure since the mid 1980s has seen a relative decline in the Human Development Index, an international ranking that determines which countries are most "modernized;" The US is 17th place in the world, despite historically being at the forefront of innovation and technology. In the "Inequality-Adjusted" Human Development Index, the United States in 28th place, between Poland and Slovakia.

The geography of the United States varies greatly from location to location. On the Atlantic coast, there are low floodplains in the Carolinas, Georgia, and parts of Florida and Virginia, while further north there are very low coastal hills, giving rise to a piedmont landform extending from Maine to central Georgia. The Appalachian Mountains are a series of very old mountains, in some places no more prominent than hills, but provided a formidable barrier for westward expansion early in the United States' history. Westward from the Appalachian Mountains lies one of the largest river systems and watersheds on Earth, the Mississippi-Missouri-Ohio River System. This system brings water from the Rocky Mountains, Appalachian Mountains, and the Canadian Shield down into very fertile and flat plains, which have been mostly turned into cropland and pasture, via thousands of miles of navigable waterways. The Great Plains and High Plains are mainly to the west of the Mississippi River. The Mississippi River ends in a large and complex delta in southern Louisiana, on the coast of the Gulf of Mexico. The Rocky Mountains, more aptly named the Rocky Mountain Cordillera, are a wide band of parallel and intersecting mountain ranges extending from central Colorado to the Pacific Ocean and from northern Canada to central Mexico, with a large depression in the middle of the area, the Great Basin. The Great Basin drains a large portion of western North America into lakes such as the Great Salt Lake, Walker Lake, and numerous smaller lakes. The Pacific Coast of the United States is filled with dramatic terrain, from the 14,500 ft Mt. Whitney to Death Valley, the lowest natural location in North America. The Pacific Coast is split into three climatic regions: Cascadia, characterized by heavy rainfall throughout the year; Northern California (or Sequoia), characterized by a dry season and filled with some of the largest trees on Earth; and Southern California (SoCal), which is much drier than Northern California but contains the vast majority of California's population, as well as that of the Pacific Coast.