The vast Salt River Valley, also known as the Valley of the Sun, stretches for miles north of the imposing South Mountains. Almost the entire range of the mountains is located in a huge city park called, appropriately, South Mountain Park. With over 16,000 acres, South Mountain Park/Preserve is one of the largest municipal parks in the United States, according to the Trust for Public Land. It boasts 51 miles of primary trails for horseback riding, hiking and mountain biking for all ability levels. The Park is the home to many forms of wildlife, including coyotes and many other small animals. The desert vegetation is typically that of the Arizona Sonora Desert. Phoenix, the principle city in the Salt River Valley, is the sixth largest city in the United States with about 4.2 million people. Unfortunately, like many large cities, Phoenix battles with air quality and the famous clear desert air is often full of haze.