"This is where the trail used to end," says Roger, our guide. "The locals wouldn't come past here."
A frisson of excitement passes through the group. We're about to enter the "dark side". For centuries, the residents of this remote valley in northern Peru believed that if you ventured past this point, you'd be cursed by evil spirits – in particular, a beautiful blonde mermaid who lived in a pool at the base of a waterfall.
It was a legend that successfully prevented one of South America's most spectacular waterfalls being discovered for centuries. Finally, in 2002, a German called Stefan Ziemendorff stumbled across Gocta Waterfall during an expedition.
Ziemendorff convinced the Peruvian government to measure it and in 2006 they held a press conference declaring the 771-metre-high waterfall to be the world's third-tallest. This has since been disputed and its ranking now ranges between fifth and 17th, depending on the criteria. Whatever its official standing, nothing will diminish the slack-jawed astonishment you feel when you first clap eyes on it.
What makes this tale all the more extraordinary is that Gocta isn't hidden away in some impossible-to-reach canyon but plainly visible from the small town of Cocachimba in the province of Chachapoyas, roughly 700 kilometres north of Lima.
It is from Cocachimba that we start the mostly downhill 5.8-kilometre hike to the base of the waterfall. The winding dirt trail passes through lush cloud forest as it skirts the edge of the valley, providing sweeping views of the surrounding mountains and teasing glimpses of the top half of the falls.
Along the way, we pass a handful of rural dwellings, a few small farms and the occasional sugarcane plantation. What we don't pass are many other tourists – a few clip-clop by on donkeys led by enterprising locals, but other than that it's just us and the birds. The valley has more than 150 species and we see red-masked parakeets and black vultures and hear the melodic flute-like call of the Andean solitaire.
Once we cross over into the "dark side", the roar of the waterfall steadily increases until we round a corner and see it for the first time in its entirety – two separate drops that in total plunge 771 metres down the face of a yawning rock amphitheatre.
A few of us decide to take a dip in the tannin-stained pool at the base, a tricky endeavour that involves clambering over slippery rocks while being buffeted by waves of spray. I'm not proud of the shriek I let out when I dive into the icy water but the humiliation is worth it. The pool provides a unique vantage point from which to observe the falls.
From here the cascading water has an ethereal quality, resembling steam, dry ice and even snow as it tumbles down in graceful waves. Sadly, Gocta's beguiling mermaid guardian never appears, but the setting is so magical and mysterious it's easy to understand how such a fantastical myth could arise.
The largely uphill hike back is a sweaty, strenuous affair, so en route we pause for sustenance at a small rustic restaurant called La Golondrina. Owner Bernardo persuades us to sample his guarapo, a distilled spirit made from sugarcane juice that he infuses with coca leaves. Full of ill-advised bravado after my swim, I down a tumbler-sized shot and the burning liquid renders me speechless for a full 30 seconds.
"After one drink, you have to ask your wife to take you home," he explains in Spanish. "After four, you're not going home at all," he adds, grinning broadly. Clearly, Gocta's mermaid isn't the valley's only evil spirit.
Rob McFarland was a guest of Peru Tourism and the Adventure Travel Trade Association .
LATAM flies to Lima five times a week from Melbourne via Santiago and daily from Sydney via Auckland and Santiago. See latam.com
The upscale Gocta Lodge has stunning views of the valley and the falls plus an outdoor pool and an excellent restaurant. See goctalodge.com
Latin America specialist Chimu Adventures can create a tailor-made Peru itinerary including flights, accommodation and tours. See chimuadventures.com