China has a diverse geography including fertile lowlands and mountains in the east, deserts and steppes in the central and western regions. Seven of the world's most important rivers originate in western China's highlands. China contains a variety of forest types and is home to famous giant pandas, golden monkeys, and other species. Hong Kong has a very dense population and was historically dependent on China but now has a highly productive economy. The Great Wall of China was constructed over thousands of years to defend against northern invaders.