The Blue Mountains is a mountainous region in New South Wales, Australia. It borders Sydney's metropolitan area, with the foothills starting approximately 50 kilometres west of the state capital. The area begins on the west side of the Nepean River and extends westward as far as Coxs River. It is one of the most accessible areas of (relatively unspoilt) highland natural beauty in New South Wales. The name Blue Mountains is derived from the blue tinge the range takes on when viewed from a distance. It’s caused by sunlight hitting particles within the atmosphere, and these particles are caused by chemicals emitted by the coniferous species of the area, creating a blue-greyish colour.