It’s a breathless summer day on the flanks of the Sella – a strikingly serrated massif in the Dolomites of northern Italy – and the twisted grey towers of rock are shifting shape as I cycle around them.

A cluster of crenellated peaks morphs into a medieval fortress guarded by imposing turrets.

A sheer grey-white limestone wall, fronted by neat vertical fissures, glistens like the pipes of an ornate church organ.

And a chain of spiked peaks evokes the armoured spine of a prehistoric monster.

On these hot, high-altitude slopes, my imagination is whirling faster than my legs.

I’m here for the early-summer Sellaronda Bike Day – a traffic-free bike challenge that takes place twice a year, in June and September, enabling cyclists to complete a circuit of the world-famous Sella massif without the roar of cars, motorbikes and buses.

The ride draws its inspiration from the legendary ski circuit of the same name.

On two wheels, the Sellaronda loop is deceptively short, at only 53km. However, with 1,637m of ascent and four iconic high-altitude passes – the 2,121m Passo Gardena, 2,240m Passo Sella, 2,239m Passo Pordoi and 1,875m Passo Campolongo – this course should not be underestimated.

Those climbs have appeared in the Giro d’Italia more than 40 times and they have regularly marked the Cima Coppi – the highest point of the Giro course.

The Sellaronda route may be circular, but its profile looks more like a cardiogram from a brutal high-intensity spin class.

I’ve been cycling up and down all day. Four high-altitude passes. Four swirling descents.

There’s hardly an inch of flat in between. The Sellaronda is not just a bike route; it’s a rock-hewn rollercoaster.

Up the Gardena path

My day begins with a breakfast of muesli and local honey at Hotel Marmolada in Corvara, a pretty town in Alta Badia, about a three-hour drive north of Venice.

At 1,500m, Corvara is loftier than the UK’s tallest peak, Ben Nevis (1,345m), but it’s dwarfed on all sides by the mammoth Sella and the hulking 3,343m mass of the Marmolada – the highest Dolomites peak.

The hotel is conveniently situated opposite the Sport Kostner bike shop, from where I pick up a gleaming red Pinarello.

I meet up with Tommaso Cominetti, a local guide from Dolomite Biking and some travelling cyclists.

Our route today is simple, with riders spinning anti-clockwise around the well-signposted loop. But this is not like other sportives.

There are no timing mats or race numbers. The road is simply closed to traffic until 4pm and you can ride at your leisure.

As a result, this event attracts an unusual mix of entrants.

When we head out of Corvara, I see, on the one hand, serious riders arrowing past us with shaved legs and beeping gadgets, who have travelled from as far afield as America and Colombia.

But there are also local families on electric bikes, many of whom will climb one pass then divert home or spend the afternoon eating ice cream in mountain-top cafes.

Tommaso warns me that the first climb, to the Passo Gardena, has the steepest slopes, with some jolts at 12 per cent.

The ascent is predictably slow, but gloriously scenic. Beneath an immaculate blue sky, we glide past flower-speckled meadows, wooden chalets, the glistening steel cables of via ferrata trails and those mesmerising white-grey peaks.

The Dolomites take their name from the special carbonate rock dolomite, which is named after the 18th-century French mineralogist Déodat Gratet de Dolomieu, who was the first to describe it.

The Gardena climb is 8.8km of twisting hairpins and soaring views, with a vertical gain of 594m and an average gradient of 6.7 per cent.

The lower hairpins of 8 per cent and 12 per cent are certainly a wake-up call and I’m soon out of the saddle.

It’s odd to be in a peloton with lithe Gran Fondo veterans, along with locals hauling dogs uphill in a buggy, but that’s part of the event’s charm.

With both lanes available, the different groups find their natural flow on the road. Just don’t panic if an octogenarian zips past you: they might (or might not) be on an ebike.

When finally I grind over the Gardena, the sweeping view of dramatic walls of rock and twisted spires on the horizon is truly spectacular. Some riders dash straight over the summit, but most stop for photographs.

The first descent is sketchy, not just because of the sharp hairpins, but because the mixed peloton ensures an awkward blend of speeds.

But just as on the ascent, the different tribes each find their niche, with the slower riders hugging the inner lane and the racers torpedoing by on the outside.

The descent is 5.9km long, with a smooth gradient of 4.2 per cent, but the most exhilarating section is a straight dash under the shadow of a colossal rock wall, which is known as Parete Fredda (Cold Wall) because it never sees sunlight.

The chilled temperatures and that steep, sinister wall of rock leave me shivering.

Hard Sella

But like a basketball hurled at the ground, as soon as we hit the valley, we bounce straight back up for the second ascent, to the Passo Sella.

My legs have frozen up, so it feels tough to get into a rhythm. The climb is 5.3km long, with a biting average of 7.1 per cent.

But we have barely cycled a few hundred metres and we’re already drinking in views of the 3,181m rock citadel of the Sassolungo.

If the Dolomites look strikingly different from other peaks, it’s because this land used to be a coral reef.

Two hundred and fifty million years ago, these peaks lay underwater in the primordial ocean of Tethys.

After aeons of tectonic activity, the coral reef became the mountain playground it is today.

And now the ancient shoals of tropical fish have been replaced with a glistening peloton of cyclists in technicolour kit.

On the final stretch, I see something furry scuttling in the valley below – a marmot? It’s much quicker than me.

At this altitude, my energy levels are melting faster than a bowl of gelato. We grab cold drinks at the summit-top restaurant and enjoy the jaw-dropping scenery.

The descent is another wild ride, with 784m of vertical loss, over 11.4km, at a speedy 6.9 per cent. I hurtle past walls of rock, mountain chalets and flower-strewn meadows in the fierce sunshine.

The descent features zippy straights, interspersed with sharp hairpins that grab your attention.

Pordoi devils

On shorter mountain sportives, it’s easy to underestimate the physical toll, and I realise I haven’t eaten enough today.

The constant climbing and high altitude make this course tough.

I wolf down an energy bar in preparation for the 2,241m Passo Pordoi, which Tommaso has been warning me all morning is seriously tough.

Maybe it’s because it comes halfway into the day’s ride, but as I grind uphill, for the third time today, I know he’s right. With 583m of ascent ahead, I feel as though I’m commencing a siege on a fortress.

The opening section weaves through a thick forest and there are long sections of straight road that suck the hope out of you.

In the 1940s, a local peasant watching the Giro claimed he saw Italian cycling legend Gino Bartali flanked by two angels, like a pair of celestial domestiques. I’d certainly welcome their heavenly assistance today.

The climb is 13km of sweaty suffering. Steep hairpins guard its upper slopes and I repeatedly haul myself out of the saddle to shift the pain around my legs.

I notice that the peloton, which has had a festive mood all day, is silent.

After what seems like an eternity, I crawl over the summit. At 2,241m, this is the highest surfaced road pass in the Dolomites.

There’s a stone memorial to Fausto Coppi, who won the Giro five times.

The Italian adored the pristine light and air of the Pordoi: “I was the first over the summit there five times, maybe because whenever I was in that area I could breathe beautifully,” he said.

The 9.4km descent begins with a sublime series of hairpins, which curl on and on. I feel dizzy but exhilarated. Most sections are smooth, but some are rough and grippy.

Then comes a speedy dart through lush mountain meadows, before we plunge towards the town of Arabba. By the time I arrive, my disc brakes are as hot as a pizza oven.

Some cyclists are taking a break in the open-air bars. I’m tempted to join them, but if I stop I’ll seize up before the Passo Campolongo – a climb that combines a charmingly musical name with some distressingly high-pitched gradients.

Killer Campolongo

It’s just a 4km ascent to the 1,850m Campolongo, but the climb averages a grim 7.2 per cent and has kicks at 11.4 per cent.

The road from this side is arguably the least scenic of the day, and soon saps my energy.

Like a brutal pyramid workout, the gradients get ever-steeper, from 5 per cent up to 8 per cent, before those vicious sections at 10-11 per cent.

When I arrive at the top, Tommaso guides us past the selfie-grabbing crowds and straight over the summit to a small restaurant, with stunning views, for some pasta.

This is not your typical sportive pit stop, but I’m happy to follow local customs.

Refuelled, we hurtle down the final 6km descent, which carves past sun-drenched meadows and mirrored lakes to Corvara.

With so many bends, it feels like riding the Moto GP circuit at Mugello. The final section, which swirls around a bowl of mountain pastures, with the peaks of the Puez Group and the Fanes Group beyond, feels like the perfect finish to the day.

After all those rapid changes in altitude, bends and the constant climbing and descending, I feel strangely light-headed at the finish – and far more fatigued than I expected.

But the greatest pain is in my neck muscles, from taking in the remarkable scenery.

As the local mountaineer Reinhold Messner once declared: “The Dolomites are not the highest, but they are certainly the most beautiful mountains in the world.”