Trekking through Puyehue National Park in Chile tests Derek Cheng's tenacity - however, magnificent views of a mammoth crater and thermal pool soaks ease the pain.

The thick forest was eating us, fleshy morsel by fleshy morsel.

I thrust an arm through the dense leaves and thorny branches, only to withdraw it - scratched and minus chunks of flesh.

I pressed forward, taking a minute to move one metre, three minutes for the next, to a spot where the only way forward was back the way I came. Any progress meant contorting my body into all sorts of shapes to fit through thin gaps.

To get to this insurmountable wall, deep inside Puyehue National Park, Chile, we had lost the path and made a ridiculous river-crossing on a fallen tree trunk. The beef on the trunk was somewhat lacking at the business end, so we had to plunge our hands into thorn bushes on the river bank to haul ourselves up. Thorns plus fleshy fingers generally don't get along.


Mother Nature was clearly unimpressed by our tenacity and gave no helpful signs as we selected our ill-fated entry point into the forest.

We soon discovered it was the wrong place but resolved to push further down a small stream, reckoning we'd eventually come to the main river and proper path, and - fingers crossed - a man eager to relieve his pack of an excess of cold beers and good food.

Turns out not many beer-carriers walk the circuit track around Mt Puyehue (meaning place of the "puye", the small native fish of the area in the indigenous Mapuche language) in the Patagonian Andes. And with good reason. Many visitors easily trek two days to the mountain's summit at 2236m above sea level, and an extra 14.5km to steaming thermal springs that are scattered along a river to the north. But most come back the same way.

The 1070m climb through lenga forest on the first day is not that punishing, and the extra 630m climb to the summit, high above the tree line, the following morning is steep but straight forward. We were rewarded with views of a mammoth ice- and snow-filled crater, while pointy white and brown peaks dotted the Andes to the north and south and a cloak of low cloud filled the valley below.

From there, it was a steady trek through a landscape of volcanic pumice and ash: the volcano spewed up acres of the stuff in 1960. It was like walking on a beige-coloured Mars. In the distance lay frozen rivers of petrified black, the remains of ancient lava flow in a more active past.

We arrived at the thermal springs to find that you had to sit where the combination of boiling bubbles and cold river flow met. Any deviation meant burning your hips or freezing your thighs.

The next morning we hiked two hours past the springs, across a river and a dried lake, to a series of bubbling geysers. As steam billows from folds of yellow and rustic red earth, sulphur water bubbles and violates the nostrils.

As our quartet split up and we watched half the group head back the same way they had come, a Polish woman, Magda, and I headed where few venture: the backside of the circuit, a gorgeous landscape that undulates over unsteady terrain.

By the time we had made our finger-puncturing river-crossing, we had already lost the track once, walking over sandy dunes only to arrive at the head of a steep waterfall. We had to back-track and pick a different way down the mountainside, as winds threw up sand twisters to remind us of our vulnerability.

The critical navigation point was where to re-enter the temperate rainforest. The forest floor soon swallowed our chosen track, but we descended the steep, often vertical stream into increasingly dense flora. I lost my footing twice and was thrown on to my back, my hands clutching desperately at the dead leaves and mud on the ground to arrest my slide.

After two hours of bashing the bush, our progress almost at a standstill, we held a meeting.

"I hate this," said Magda.

"We have to go back. This forest will kill us."

I agreed.

We re-emerged from the forest, some 12 hours after we had started the day, tired and reeking of sweat and tree sap. I pitched my tent as the sun set over the broad-shouldered Mt Puyehue, and we were soon enveloped in deep sleep.

The following morning - day four - thirsty and with bodies aching, we packed up and reviewed our small Lonely Planet map. It was like trying to navigate the Amazon with a map of the world off a cereal box. But something about the line marking where the track should be compelled us to retrace our steps towards the volcano in the hope of finding ... something.

Spirits were low. Steps were slow.

And then, from a great distance, I spotted a bamboo stick, the first we had seen for hundreds of metres.

Our strides instantly lengthened, our feet ceased to ache and, soon, we were in the forest. On the right track.

The forest, replete with tall beech trees resembling the west coast forest of New Zealand's South Island, had an invaluable trait - refuge from the swarms of giant, blood-sucking horseflies. About twice the size of the normal housefly, they hatch sometime at the start of the warmer season, usually at the end of December, and wreak havoc for 60 days or so before dying.

By 3pm, our feet battered and our arms flailing about in a constant dance to ward off the flies, we arrived at the restaurant at the park's entrance, having covered about 85km.

"We got lost," I announced to the chef at the restaurant. But he seemed more intent on heaping praise on us for completing the circuit in three-and-a-half days than listening to what a potential death-trap the circuit is.

The fly-swatting dance continued as I stood roadside, trying to hitch a ride back to the Chilean town of Osorno, one of the most boring towns in South America and, to me, noteworthy as I had stored a bag there. After 20 minutes, a family of four from Santiago pulled up. They were going to the nearby thermal springs and offered us a ride.

"We have a cabin on the lakefront," the father said.

"Would you like to stay with us for the night?"

The thermal springs were somewhat different to the ones in the river on the trek: a large shallow swimming pool, full of lounging adults and splashing kids. Entering made all the scratches and cuts on my body scream out in agony.

After the soak in the pools, we went back to the cabin and had a bland Chilean supper of bread with cheese, followed by another South American tradition - screaming at football on the TV.

The next day, I set off to hitchike to Osorno. I was only a few minutes on the road before a ute stopped and picked me up.

The girl in the backseat was keen to practise her English.

"What are those marks on your arms?" she asked curiously.

"The forest attacked me."

She looked at me in confusion before I explained that I had walked the back of the Puyehue circuit.

She understood instantly.

Getting there: LAN Chile flies from Auckland to Santiago, with connections throughout South America.

Osorno is the closest town and about 100km from this four-day trek in the Puyehue National Park. Buses run twice daily, the first leaving at 10.30am, to the park entrance and cost 9000 Chilean pesos ($29) one-way for two people.