There were no guarantees… but some things you just have to try. The Ubaye Valley had haunted me ever since I passed through it while following the legendary Route des Grandes Alpes, created by linking together rugged tracks and passes constructed by the military to access a string of remote, high-altitude forts defending the mountainous French border.

Head south from Briançon today and you enter Ubaye over the famous 2109m Col de Vars, but it’s not the original pass — that was further along the valley, and much higher. Long since abandoned, the old trace (including a remote and mysterious tunnel) scaled the wild, rugged Massif du Parpaillon and still appears on larger-scale maps, Michelin adding a stern warning (in red letters): ‘Passage Uncertain’. Was it, I wondered, still possible to follow it from end to end? I was determined to return and find out.

Initial reactions of the local tourism people sound ominous: “It could be possible, but there’s a problem with the tunnel — most of the year it’s frozen up with ice and snow…”. Then there’s a ray of hope: “If you can come after the middle of June it should be open, until the snows come again.”. So, the time-frame is set. The itinerary, too, becomes clearer when I receive a brochure promoting the pleasures of ‘le VTT’ (vélo tout-térrain) in and around the valley. Doing it by mountain bike looks straightforward enough, until I pinpoint the relevant itinerary: ‘le Parpaillon 37km’. Doesn’t sound a lot but it’s graded Black, which, as skiers will already know, equates to “trés difficile”. Gulp. The tourism people, though, assure me that the grading is merely a result of the distance having been calculated for an aller-rétour return trip, so I decide to go for it. But just the one way.

After months of waiting, and increasingly lengthy training rides, I find myself back in Ubaye, renting a convincingly rugged full-suspension bike and feeling distinctly uneasy about what lies before (and above) me. The Route du Parpaillon originally served the Fort de Tournoux, a vast military complex defending the eastern end of the valley where it meets the road from the Col de Larche and the Italian frontier in the village of La Condamine-Châtelard. From here a smooth tarmac road launches through a series of tough hairpin bends to over 1800m, passing the family ski station of Sainte-Anne. Mercifully, the Lost Route concept gives me the perfect excuse to drive this gruelling 6km section, unlike the squads of serious road cyclists who come specifically to attack the tough Alpine cols, one after another. The tarmac finally runs out beside the tiny Chapelle Sainte-Anne, where it’s time to transfer to two wheels.

The old route begins as a gravel track among lush wildflower meadows, but soon enters dense, echoing forest and climbs steadily up the side of a deep, curving valley. A few minutes’ determined slog brings me to the Pont du Bérard, an idyllic spot where an inscribed stone beside a craggy cascade records, among other things, the altitude (currently 1841m) and distance to the tunnel: 9km 995m. Doesn’t sound a lot. Until, that is, my rear brake pads announce their sudden demise with zero effectiveness accompanied by a dull grinding sound. Given that it’s a Sunday and the bike shop won’t be reopening until Tuesday, I have no choice but to press onward.

Progress now becomes slower and the pulse rate faster on a steep hairpin climb through the tree-cover, from which I finally emerge to confront a cheerful cabin equipped with a gently-humming generator. Unlike the small group of walkers following the GR 6 footpath, I resist the temptations of the Buvette Grill le Petit Clausis and instead press on over a rugged stone and timber bridge spanning the Ruisseau du Parpaillon, today little more than a small stream. Now things really open out, the rough track ahead laid out like a dull grey ribbon following the contours of the steep valley sides past the grandly-named Cabane du Grand Parpaillon (2031m), the only obvious man-made feature. After pausing for a drink beside this simple stone refuge, I set off towards a small dot approaching from the opposite direction. Minutes later a breathless cyclist wishes me ‘Bon courage!’ as he rattles down towards the floor of the valley. We’ll see.

Uphill progress, not surprisingly, is painfully slow, the combination of sun and altitude compounding the effort required. The scenery, though, is ever more amazing, with few clues (apart from the concept of constructing anything in such wild and remote terrain) as to the route’s former importance. Then I pass over a short section surfaced with intricately aligned stonework. It’s hard to imagine the whole Route having been constructed so painstakingly, and even if it had, surely other sections would have survived? A mystery.

Higher still the track becomes more tortuous, finally tightening into a seemingly endless hairpin ascent, repeatedly crossing and re-crossing a deep gully gouged by torrents of rain and melt-water cascading down to the valley floor far below. Each crossing is via a timber bridge of dubious structural integrity. Skirting the holes gaping alarmingly in their sagging, sun-bleached timbers concentrates the mind, as do several sections awash with unstable rocky debris sliding down the mountainsides. ‘Off-road’ is putting it mildly, and suddenly the rumours I’d heard of the itinerary having actually been used as a stage back in the early days of le Tour begin to sound pretty far-fetched. Eventually, though, I round what turns out to be the final bend and glimpse in the distance the legendary Tunnel du Parpaillon. The sight inspires a final burst of energy, acknowledged by a stirring round of applause from a large group of Italian long-distance hikers taking a picnic break.

After satisfying their demands for a photo-call, I collapse, take in the scenery and consume anything edible remaining in my back-pack. Inscriptions on the stonework around the tunnel entrance record its constructors (le Génie, or military engineers) and its altitude. At 2643m, or almost 9,000ft, this is almost certainly Europe’s highest tunnel, and certainly feels like it. The spot is both uplifting and yet desolate in its remoteness, with little greenery able to survive on the stark, almost lunar terrain between the tunnel and the Col du Parpaillon above. Not surprisingly, keeping the Col open (and maintaining the Route’s viability) during winter proved impossible, so the tunnel was bored 60m below the ridge and opened in 1901.

Peering into the entrance today, after years of abandonment, sends a shiver down the spine. A very few metres beyond a pair of huge steel doors, all is total darkness, and there’s an eerie sound of dripping.

But there’s no going back, so after donning a thermal fleece and a supposedly long-range LED head-light, I select a low gear and launch off into the void at a steady pace, so as to keep moving through the deep pools of water below and to aid stability when I hit the inevitable sheet ice. As expected, the cold inside is glacial and the darkness total, for the tunnel’s course is not straight, but arcs throughout its 468m length. Result: it’s impossible to see the exit until you’re well committed. Eventually, though, a tiny white dot appears, and focusing upon it goes some way to blocking out thoughts of the 200ft or so of solid (and not-so-solid) rock poised overhead. Sure enough, there’s ice, but somehow I keep moving.

Once safely out in the warm sunshine again the sense of elation is unbelievable. A young Italian cyclist and his girlfriend, looking doubtful, ask me if its safe to ride through, but pale visibly at the mention of ice and decide to bottle out. For me, though, the rite of passage is behind me, and I focus instead on the beauty of the very different landscapes unfolding ahead. From here it’s all downhill, an epic plunge between mountain pastures ablaze with Alpine wildflowers, and having only one usable (front) brake means I have plenty of time to take in the scenery. Eventually, after crossing a fast-flowing stream and passing a few rugged stone cabins and timber crosses, I hit the tree-line once again, entering a silent conifer forest sheltered by the surrounding mountains. Minutes later the gravel gives way unexpectedly to tarmac before passing a steep, rocky track signed to Crévoux. I continue down more sedately to cross the Torrent de Crévoux, emerging into a broad valley above the starkly authentic-looking hamlet of La Chalp. The journey is almost complete, apart from a final streamside descent beside to the Vallée de la Durance. In the village of Crévoux I discover a time-worn painted plaque proudly recording the completion in 1891 of a major section of the Route Militaire du Parpaillon by the 6th Group Alpin. Thanks to their great adventure, it was still possible to undertake mine.

So, would I do it again? You bet — there’s something haunting about the wildness and mystery of it all which draws you back. Okay, next time I’ll probably do it in the opposite direction, for a different perspective on things along the way. But the south-to-north approach really is the Big One - and, sooner or later, you have to do it.