Formed thousands of years ago by the activity of the Pivka River, the Postojna Caves known today cover a network totalling more than 12 miles (19 kilometres) - which makes it the largest karst rock cave in Europe. The caves were discovered in the late 18th century and were never mined, so they could quickly be opened to the public. The first visitors came in 1819.
One of the elements that make caves so attractive to visitors is the formations (including stalagmites and stalactites) resulting from deposits of calcium carbonate. Another reason for the caves' early success was because they were the first to use a number of features. For example, it was lit with electricity in 1884 - preceding even Ljubljana, Slovenia's capital. Railway lines were laid in some of the caves in 1872, at a time when many working mines did not use rail traction. The labyrinth of caves can still be entered on the small electric railway, which takes you into the heart of the caves, allowing you to explore the larger caverns on foot.