Further investigation reveals a hole and, after using our camera flashes to illuminate the entrance, we discover a tunnel. We exchange a "what the hell, we've come this far" look and I follow him in. On the other side is another flight of breath-sapping steps but the end is finally in sight. We edge around a large boulder, climb a small wooden ladder and join a handful of other elated climbers on what feels like the top of the world.
The view from the top of Wayna Picchu is, quite simply, one of the planet's most inspiring. Now and again the clouds engulfing us disperse and we get fleeting glimpses of Machu Picchu, the fabled lost city built by the Incas down below. We're surrounded by tree-covered mountains soaring up from valleys and rivers barely visible they're so far away.
There are just five of us here, perched precariously on a summit of huge granite boulders, taking turns to photograph a vista we'll never forget.
It's enough to make a grown man cry. Thankfully, Rob doesn't notice.
Of the thousands of camera-wielding tourists who traipse over Machu Picchu each day, only a small number tackle the strenuous, one-hour climb up Wayna Picchu - the ominous, cloud-obscured mountain looming in the background of almost every Machu Picchu picture.
I'd read about the ascent in Lonely Planet but had mistakenly thought it was referring to the smaller, much more climbable-looking hill just at the back of the ruins. Discovering it was actually the towering monolith whose peak is almost always shrouded in cloud, I contemplated feigning some sort of seizure.
Access to the path is via a manned checkpoint at the back of the ruins. A maximum 400 people per day are allowed to attempt the ascent and everyone must start before 1pm and be back before 4pm. People have died on the way which might explain why the guard advised us against tackling it given the wet, slippery conditions.
The climb is not technically difficult - you just follow the steep stone path pretty much to the top - but the altitude makes you feel like you're conquering Everest. Two and a half thousand metres above sea level and the air contains significantly less oxygen.
The key is to take it easy and go at your own pace. We took plenty of rest stops and were continually encouraged by the climbers who were making their way down. Halfway up, I stopped an American guy and breathlessly asked him whether it was really worth all the effort.
He grinned and in a heavy Texan drawl simply said: "It's awesome man!"
The feeling of achievement once you hit the summit is incredible. I instantly forgot every lung-bursting step once I was confronted by the scale of the panorama out there.
From here it's easy to see why Machu Picchu is Peru's number one "must-see" attraction. It's one of those places that manages to amaze on so many levels that the whole experience can feel overwhelming. Most pictures tend to focus on the ruins so it's difficult to appreciate the grandeur of the surrounding scenery. All around, mountain ranges rush down into far-off valleys and wisps of passing cloud add a mysterious, ethereal feel.
While the scenery is breathtaking, what sets Machu Picchu apart from other world wonders for me is that it's man-made.
It's almost impossible to comprehend how the Incas built this impossibly located city more than 500 years ago. Manoeuvring and carving stones weighing several tonnes by hand at this altitude is an astonishing feat.
Machu Picchu's trump card is that it was only discovered by Westerners less than 100 years ago. How stunned must American historian Hiram Bingham III have felt in 1911 when he stumbled across this ancient city that had escaped detection for half a millennium.
In our age of scientific exactness, there's something romantic about the fact archaeologists still don't know exactly when or why it was built.
Despite Machu Picchu's historical and archaeological significance, visitors enjoy unhindered access to the site. UNESCO has warned the Peruvian Government the ruins will be irretrievably damaged if visitor numbers are maintained so this is unlikely to always be the case.
While more stringent controls are no doubt required, my only hope is that authorities continue to allow people to climb Wayna Picchu and experience the breathtaking view from the top.
Rob McFarland travelled as a guest of Tucan Travel and Aerolineas Argentinas.
