The disappointment is enough to make me feel ill. I have spent two days getting here, having negotiated rough mountain roads by motorcycle, and spent a considerable amount of money on this once-in-a-lifetime experience: seeing Machu Picchu. But just as I arrived, a thick fog enveloped the mountain range. I can barely see the slippery stone steps I'm climbing, never mind the ruined city spread out below me.
I stand in a huddle of tourists who rushed to catch the first bus, at 5.30am, to the historic site. We all mutter variations on a theme: "Such a shame"; "Well, we can still explore the city, I guess"; "I can't believe it, after I've come all this … wait. There it is!"
As the sun rises, the fog clears, and Machu Picchu, the mysterious "lost city of the Incas", slowly reveals itself. Built at 2,430 metres above sea level in the middle of a tropical mountain forest, it sits snugly in a saddle on the eastern flank of the Peruvian Andes. The granite walls and terraces of the city are flooded by sunlight in what is an awesome display. There is no muttering now, making the click of camera shutters seem all that much louder.
Standing on the edge of the city is a dizzying experience, with the mountains dropping away to deep and narrow valleys that have been cultivated continuously for well over 1,000 years. From atop the city walls can be seen a meander in the Rio Urubamba, where construction workers are building a hydroelectric dam.
No one is quite sure why Machu Picchu exists. It was built sometime in the mid- to late-1400s, an urban architectural exclamation mark of the Inca empire at its height. Perhaps it was ceremonial, perhaps it was a summer retreat for rulers, or maybe the Inca planned it as a last stronghold.
The city appears to be divided into quarters: one for farmers, near the massive cultivated terraces and hanging gardens; one for industry; one for royals; and one with temples, for the religious side of life.
On Unesco's World Heritage list, Machu Picchu is one of the most visited archaeological sites in the world, and most of those who come do so via Cuzco, the historic capital of the Inca empire and a significant tourist destination in its own right. From the colonial grandeur of Cuzco's cobbled streets, visitors hop on a scenic, zigzagging narrow-gauge train that trundles through the Andes and deposits its passengers at the tourist way station of Aguas Calientes, at the base of Machu Picchu, which is also the name of one of two mountains either side of the saddle.
There is another, more adventurous approach to Machu Picchu, though. It involves a four-hour bus - or motorbike - ride along a twisting, climbing mountain highway to the town of Santa Maria. From there, the road turns to dirt and rock, clinging to the mountainsides and squeezing under overhanging cliffs for another 30 kilometres before arriving at the mountain village of Santa Teresa. Once there, you can hop on the train to Aguas Calientes, a journey of 30 minutes, or, as most people who take this route do, you can follow the train tracks on foot, enjoying the rushing white-water river and mountain views as you do so.
Whether they arrived by train, motorbike or on foot, most visitors spend at least one night in Aguas Calientes, even though the town exists only to fleece tourists.
MY ALARM GOES off at 4.45am, one of a chorus that can be heard through the thin walls of the guesthouse. Everyone wants to be first up the mountain, like skiers competing for the first run down after a snowfall. The street is soon filled with a stream of tourists, bundled up in warm jackets, fingers wrapped around steaming cups of coca tea, all heading to the bus station, for the journey over the last few kilometres to the fabled city.
Machu Picchu, covered in jungle foliage, was unknown to the outside world until July 24, 1911, when Yale University historian and explorer Hiram Bingham was led to the site by a local herder boy. The "lost city of the Incas" is lost no more: the bus I'm on is packed tight.
Machu Picchu gets an average of 1,800 visitors a day, with a maximum of 2,500 allowed in, more than twice the Unesco-recommended limit. Aguas Calientes has grown into a town of 4,000, with hotels and restaurants built in a messy, unsightly jumble. All that activity is causing severe soil erosion and damage to vegetation, which led the World Monuments Fund to place Machu Picchu on its 2008 Watch List of the 100 Most Endangered Sites in the world. In the same year, Unesco said there were "urgent problems with deforestation, the risk of landslides, uncontrolled urban development and illegal access to the sanctuary".
An incident in 2000 highlighted the damage being caused by the encroachment of modernity. The Peruvian beverage company Cusqueña was filming a beer commercial at the site when a camera crane struck the intihuatana, or "hitching post of the sun" (a carved stone pillar that is part of the most important shrine at the site), breaking off a piece.
Nevertheless, in 2010, Peru's Congress approved construction of an access road to the remote ruins, opening the doors to even more tourists.
And they keep coming.
"I never thought I'd see this place with my own eyes," says one of two greying women chatting in hushed tones as they sit on a stone ledge overlooking the terraces. "I'm so glad I made it here while I could still hike up these terraces."
"I'm here for my mother, who always dreamt of seeing it but never did," says the other.
A short distance away several young backpackers from California have formed a circle and are doing yoga beside a 15th-century watchtower. They have just hiked the Inca Trail, a gruelling five-day trek along a 43-kilometre route that some believe was built for holy pilgrimages.
From a quiet spot overlooking Machu Picchu it's possible to absorb its ancient wonder and take inspiration from the meticulously fitted stone walls, built without mortar. Below, tourists are crawling over the city, exploring its roofless houses and narrow alleyways. Each has made a special journey here, some harder than others, some after a lifetime of dreaming, but all are drawn by the mystery and grandeur of an ancient mountaintop city the purpose of which remains unknown to anyone who is alive today.
Getting there: KLM (www.klm.com ) flies daily from Hong Kong to Amsterdam's Schiphol Airport, and from there to Lima's Jorge Chavez Airport. There are many flights (operated by Lan and StarPeru, for example) from Lima to Cuzco (a flight time of just over an hour) every day.