There is an abandoned mine in Slovenia where you can ride mountain bikes (3 photos + 1 video)
The route is called “Black Hole” - it is 10 km long and 150 meters deep. Arrivals are only possible with the accompaniment of a professional guide. 
Carinthia is a real paradise for lovers of mountain hiking and those who conquer them on a mountain bike, and Mount Pec, within which ancient legends live, is a part of this paradise. The most famous legend among Slovenians is the ballad about King Matjaž, who always did good, and now he sleeps under the mountain, while his beard, growing, wraps around the table. However, King Matjaz is not the only secret of Mount Pec; its underground world is also the abode of myths and legends.
Between Mount Peca and Mount Urshlia, ore deposits were discovered in ancient times. Over more than three centuries of mining development, 19 million tons of lead and zinc ore were mined and 800 kilometers of drifts were dug, but this mine, work in which was completed 20 years ago, still serves in a unique way: since 1997, part of the development was open to tourists. You can arrive here by train, and those who prefer adventure can go underground by bicycle. 
Although a mountain bike is primarily intended for riding in open terrain, in Carinthia you can ride it through the mountains. And this is in the most literal sense of the word. You just need to dress warmly, stock up on leggings, a helmet and a miner’s lantern, and in the company of a guide, drive a little over five kilometers along the underground route leading from one valley to another. Do you feel what adventures await you? The route passes through trenches that do not pose a danger; the overall rise does not exceed 15 meters, however, the recesses and mounds of former trolley transfer points will significantly complicate the trip. This entire adventure lasts two and a half hours. 
