The most bizarre thing isn’t that I’m cycling in a ceaseless semi-darkness, up a narrow passageway, the air so dank it feels as if my lungs are stuffed with cotton wool. Rather, the craziest part of this journey is that my endeavours are occurring an incredible 500 metres underground on a one-of-a-kind bike tour of deep inside Mount Peca. This 2,100-metre-high mountain, which straddles the Austrian-Slovenian border, is situated in the stunningly beautiful Karawanks mountain range, that is part of the Alps. I’m in the latter Central European country – around 100km from the capital city, Ljubljana – on an extraordinary ride that an eco- travel outfit, Bike Nomad, introduced more than 15 years ago. Nasa opens the door for private space travel – but what’s the catch? Fortunately, I am not alone on a 5km trek through what turns out to be the gargantuan disused lead, zinc and wulfenite mine of Mežica, as I can get lost in the equivalent space of a shower cubicle. My 26-year-old guide, Miha, leads the way as we proceed into what at times seems like the bowels of the Earth. Before we set off, I don a helmet with a small lamp – none of the route is illuminated – and an orange boiler suit to help stay warm (the average temperature is only 10 degrees Celsius) that makes me look like a Netflix serial killer. There is also a quick safety briefing. The entrance to this now-defunct quarry is utterly inconspicuous – a less than two-metre-high green wooden door. Once inside and saddled up, Miha and I progress along at a sedate pace of between 7km/h and 10km/h. Our whole outing occurs on level four (out of 23) – an enjoyable slew of abandoned narrow channels once used by the miners that in the main are relatively flat. In fact, it’s in no way an arduous trek so Bike Nomad say that only a basic fitness and skill requirement is necessary (the age limit is from 10 to 70-plus). Despite the ease of the excursion, at one point, Miha exclaims: “You should always stick just a metre and a half behind my back wheel, and try not to wander off”. I get his drift, as even in my short stay I occasionally get disoriented in the inky, eerie stillness; and realise that if I did go missing in this humongous 800km tunnelled labyrinth, one could probably expect to read newspaper headlines declaring the Loch Ness monster had been found before I ever resurfaced. How space travel dreams sparked Jeff Bezos’ feud with Elon Musk What makes this trip even more memorable is that at four to five places where we stop, Miha tells me about the extensive history of the mine (and he should know, as both his father and grandpa were miners here). “The Romans were the first people to dig for lead ore at this location, and from 1665 up until the mid-1990s Mežica existed as a quarry where zinc and wulfenite crystal were also extracted.” My guide leads the way as we proceed into what at times seems like the bowels of the Earth Xav Judd We journey deeper into a mountain composed of limestone, encountering mine shafts and countless hollows so big they could comfortably swallow an entire building. Peering into one, I hear a noise and get spooked. My imagination runs wild until Miha tells me it’s probably only the bats that swoop around down here. Then he reveals more about the quarrying: “It was really monotonous, depressing, back-breaking work.” Indeed, at a certain spot where we dismount, I am shown a “bison” – an unwieldy, 60kg digging tool some of the labour force had to carry – including up 600 stairs. “It was often an extremely dangerous job too. For instance, explosives were frequently used to excavate the tunnels,” continues Miha. Get a taste of the Highlands from aboard the Belmond Royal Scotsman Even though I consider myself to be quite a decent cyclist – doing a few kilometres per week – I’d be as crazy as a sumo wrestler trying to enter a high board diving contest if I’d opted for the Black Hole excursion. Why? Because this exceptionally technical, 10km subterranean trail, which is also in the Mežica mine, and is 10 out of 10 for difficulty (and then some). It’s not surprising, then, that Bike Nomad states that what is perhaps the ultimate unusual two-wheeler challenge is only open to advanced or expert riders. And on this ultra-steep course, which traverses five levels and descends some 150 metres, expect to face minimal light, twists and turns, passages where there isn’t room to swing a cat, and an almost vertical drop. Even though I consider myself to be quite a decent cyclist – doing a few kilometres per week – I’d be as crazy as a sumo wrestler trying to enter a high board diving contest if I’d opted for the Black Hole excursion According to Suzana Fajmut, who set up Bike Nomad with her husband Dixi (they also organise various outdoor pedalling trails and run a quaint eco-cycle hotel called Koroš), a fair share of their clients are millennials. She adds that they have been welcoming more visitors from South Korea and the rest of Asia in recent years. If any travellers seek similar underground experiences, it is possible to join a 90-minute guided cycle tour of the Sibbergroeve, an old (but still active) marlstone quarry in Valkenburg in the Netherlands. Starting at 40 metres below the surface, participants ride through an atmospheric 8km – of a total 70km – pale-yellow labyrinth. It is quite a hands-on extravaganza, as one often has to duck in the saddle due to low ceilings or navigate winding corridors that are occasionally decorated with charming charcoal drawings. The world’s most Instagram-friendly luxury wellness resorts Another electrifying option for pedal enthusiasts is the Louisville Mega Cavern in the US state of Kentucky, where the get-your-wheels-dirty action occurs at a depth of 30m. Not only is this former limestone mine, which was initially excavated as part of a government works’ programme after World War II, the only underground, purpose-built, bike park on the planet, it is also the largest such indoor facility of its type. 6 of the best new hotels and resorts in Maldives It is such a ginormous expanse (close to 30,000 square metres) so it is no wonder they have room for 45 trails, mix and match jump lines, pump track sections, a dual slalom, and BMX and cross-country courses (although this whole complex is currently going through a redesign, meaning the attractions might vary). Back to my mini-peloton of two (normally there are roughly 10 members in a group): there was no maillot jaune (yellow jersey) – the top award to the overall winner of the Tour de France – at the end of my leisurely exploits; but, my more than worthwhile prize was taking part in a fascinating, entirely unique two-wheel experience. More challenges in Slovenia Paragliding Forget about the ongoing spate of Hollywood superhero movies. If you really want to fly, nothing’s more exciting than paragliding. And there’s no more romantic setting than in the vicinity of the eponymous emerald-coloured lake and medieval hilltop castle of Bled. So sit back and relax in your harness with your pilot, as your craft soars like a kestrel at speeds approaching 40km/h. Canyoning Why opt to do one specific high-octane pursuit when you can do a whole bunch of them on a single excursion? That’s what’s in store for the adventurer who goes canyoning, as timing, agility, strength, determination and courage are required to abseil, climb, jump, scramble and swim, among others. White-water rafting Take the plunge and get horribly wet in the most delectable way, in the translucent whirl of aquamarine that constitutes the River Sava. Surrounded by the postcard-pretty Slovenian countryside, learn the basics of this exhilarating pastime, and then enjoy the heart-pumping-in-overdrive thrill of hurtling downstream. Useful contacts: Bike Nomad for underground biking tours, including the challenging Black Hole Trail; OUTdoor Slovenia for paragliding and canyoning; Slovenian Tourist Board ( www.slovenia.info/en ) Want more stories like this? Sign up here . Follow STYLE on Facebook , Instagram , YouTube and Twitter