The Blue Mountains is a mountainous region and a mountain range located in New South Wales, Australia.
The region borders on Sydney’s metropolitan area, its foothills starting about 50 kilometres (31 mi) west of the state capital. The public’s understanding of the extent of the Blue Mountains is varied, as it forms only part of an extensive mountainous area associated with the Great Dividing Range. Officially the Blue Mountains region is bounded by the Nepean and Hawkesbury rivers in the east, the Coxs River and Lake Burragorang to the west and south, and the Wolgan and Colo rivers to the north. Geologically, it is situated in the central parts of the Sydney Basin. The Blue Mountains Range comprises a range of mountains, plateau and escarpments extending off the Great Dividing Range about 4.8 kilometres (3.0 mi) northwest of Wolgan Gap in a generally southeasternly direction for about 96 kilometres (60 mi), terminating at Emu Plains. For about two thirds of its length it is traversed by the Great Western Highway and the Main Western railway line. Several established towns are situated on its heights, including Katoomba, Blackheath, Mount Victoria, and Springwood. The range forms the watershed between Coxs River to the south and the Grose and Wolgan rivers to the north. The range contains the Explorer Range and the Bell Range.
The Blue Mountains are a dissected plateau carved in sandstone bedrock. They are now a series of ridge lines separated by gorges up to 760 metres (2,490 ft) deep. The highest point in the Blue Mountains, as it is now defined, is an unnamed point with an elevation of 1,189 m (3,901 ft) AHD, located 7 kilometres (4.3 mi) north-east of Lithgow. However, the highest point in the broader region that was once considered to be the Blue Mountains is Mount Bindo, with an elevation 1,362 m (4,469 ft) AHD. A large part of the Blue Mountains is incorporated into the Greater Blue Mountains Area World Heritage Site, consisting of seven national park areas and a conservation reserve.