Introduction

The United States is the world's third largest country by land area,but this is disputed as some sources list the People's Republic of China as larger than the U.S. The disputed ranking stems from ongoing debate over the political status of Taiwan, without which China is smaller than the United States.
The contiguous U.S. is bounded by the North Atlantic Ocean to the east, the North Pacific Ocean to the west, Canada to the north, and Mexico to the south. Alaska is bounded by the Pacific to the south, the Arctic Ocean to the north, and Canada to the east. The island state of Hawaii is situated in the Pacific, southwest of the North American mainland.
The landscape of the United States is extremely varied. In the east, rolling hills and temperate forests prevail, transitioning to prairies, boreal forests, and the relatively young Rocky Mountains in the west, with savanna and desert—including the Grand Canyon—among the more arid landscapes in the southwest. The northwest has the highest concentration of active volcanoes in the U.S. In the south, especially in Florida, subtropical forests and marshlands can be found. In Alaska, prominent mountain ranges rise up sharply from broad, flat tundra plains; numerous volcanoes can be found throughout the Alexander and Aleutian Islands south and west of the Alaskan mainland. Hawaii, a chain of tropical, volcanic islands, is a popular tourist attraction. Other major hydrographic features include the five Great Lakes and the St. Lawrence River and seaway in the northeast bordering with Canada. In addition, the Mississippi-Missouri River, which snakes through the central U.S., is the world's fourth longest river system.
The United States is frequently affected by a large variety of natural disasters. A common threat is the tornado, especially in the South, the Great Plains, and other parts of the Midwest. Other problems include tropical cyclones along the Gulf of Mexico and Atlantic coasts, floods, and earthquakes, particularly in southern Alaska and around California. Droughts too are prevalent, with the worst being the infamous Dust Bowl of the 1930s. Recently, the western U.S. experienced widespread drought from 1999–2004, and signs of a major, long-term drought across the Great Plains have developed.
The diversity makes the United States a wrld unto itself.