North America is a continent in the Earth's northern hemisphere and western hemisphere. It is bordered on the north by the Arctic 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. It covers an area of about 24,709,000 square kilometers (9,540,000 sq mi), about 4.8% of the planet's surface or about 16.5% of its land area. As of July 2007, its population was estimated at nearly 524 million people. It is the third-largest continent in area, following Asia and Africa, and is fourth in population after Asia, Africa, and Europe. North America and South America are collectively known as the Americas.