Jenolan Caves - Australia
Jenolan Caves in Australia’s Blue Mountains (~50 km west of Sydney) are the oldest discovered open system in the world, according to Australian geologists. It was also the first cave to install electric lighting in 1880. There are 40 kilometers of known passages. Though there is no public transportation to the caves, they are a big tourist attraction, bringing in 250,000 visitors every year, and the area around them is developed with hotels and restaurants. There are 10 caves that are developed for self-guided and regular daily walking tours, but much of the system is undeveloped and only explored by cavers. Photo: Peter Kennett