I'm having second thoughts. The steep stone path we've been slowly climbing for the last half-hour has disappeared and we're standing on a small section of terracing with terrifying thousand-metre drops on three sides.

Maybe this wasn't such a good idea. Perhaps we should have heeded the advice of the security guard who told us not to carry on.

Suddenly, Rob (the only other person on the tour foolish enough to attempt this with me) spots a small arrow pointing into what appears to be a sheer rock face.

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.