Exploring the spectacular Hang Son Doong river caves in Quang Binh province, Vietnam

In the darkness above, thousands of swifts nest in nooks and crannies, 140 metres above our heads. Their high-pitched chirps ripple and echo endlessly through the 200 metre-wide cavern. Someone shines a torch above us and, gazing through the flywire screen of my tent door, I'm suddenly reminded of the sublime dimensions of my home for the night: Hang En, a 1.6 kilometre-long limestone river cave, deep in the jungle of Phong Nha-Ke Bang National Park.

But I'm no amateur potholer, let alone a professional speleologist. What am I doing in this prehistoric fantasyland, used by Hollywood to depict Neverland in the 2015 film Pan?

Had you asked me about Vietnam a few weeks ago, like most people I would have enthused about sun-bathed rice paddies, wonderful food and perhaps the Vietnam-American War. It was a chat with a British expat in Ho Chi Minh City that set me on a course underground. She had been living in Vietnam for past four years and, she said, spending a night in a cave was one of the most amazing things she'd ever done. You must go if you can, she said.

So, I went. Off to Phong Nha in Vietnam's central Quang Binh province, known for its national park and a network of stunning river caves. While several companies offer day-trip boat tours to popular Phong Nha Cave, only a few are equipped to take people to more remote places. I chose the largest and most reputable of these, Oxalis.

Oxalis's founder, Nguyen Chau, is a young-looking 41-year-old who left his impoverished village at 20 in search of a better life in Ho Chi Minh City. He studied tourism, worked as a tour guide around Vietnam and eventually found himself involved in social compliance auditing, corporate responsibility and sustainable development. In 2010 he returned to his home town and shortly afterwards it was devastated by a 100-year-flood. "People lost everything," he says, nursing a soft drink at his company's cafe-restaurant on the banks of Phong Nha's Song Con river. "After the flood, I realised, this is something we need to do, to develop the area."

And so Oxalis was born. Chau teamed up with British amateur caving couple Howard and Deb Limbert, who had been frequent visitors to Phong Nha, and together they built a successful adventure tour business, strategically designed to protect the environment and benefit local people. Working closely with the government, Oxalis restricts visitors to 5000 each year and employs more than 300 Phong Nha people directly. The surrounding homestay economy also gets a boost and Oxalis provides hospitality training to villagers through its own accommodation business, Chay Lap Farmstay.

The tour I choose, to Hang En ("Swift Cave"), is limited to 16 people. It begins on a bus at 8.30am and, as the guests climb aboard (Brits, Americans, Danes, Irish, a Spaniard, a Ukrainian, Dutch and one Vietnamese), I notice that at 42 I'm by far the oldest in a group otherwise made up of twenty- and early thirtysomethings.

At Oxalis headquarters we are issued with water bottles, walking boots (for those who need them) and briefed on the dangers we may face. Cave tours vary in physical difficulty, our guides Vuong, 28, and Vo, 29, say. Hang En is classed as "moderate" for "physically active guests", involving 22 kilometres of hiking and about 30 river crossings. Beware of leeches, snakes and, they say, clicking over the slides to show a beautiful, glossy-leafed plant, "the worst": poison ivy, whose itching hell takes five to seven days to subside. As each of us sign the lengthy release form I'm sure I'm not only one who fleetingly wonders if it's too late to pull out.

Next, a 40-minute bus ride along the Ho Chi Minh trail, through stunning mountains and a military guarded boom gate, marking our arrival in Phong Nha-Ke Bang National Park. This 885-square-kilometre Unesco World Heritage Site is home not only to the oldest karst (limestone) mountains in Asia (400 million years old), but 23 animals listed on the world "red book" of threatened species.

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Before long we are walking single file through the thick, dark jungle, descending a living staircase of gnarled roots, clay and leaf litter. Unseen insects chirp and scratch in rhythm around us. Within five minutes my shirt is soaked with sweat and the group has found its natural pecking order: fit twentysomethings up the front; me, and a petite, 20-year-old Vietnamese literary student, rounding out the back.

The track begins to angle down, 45 degrees. It's tough on the knees but after an hour or so, relief comes in the form of a flat riverbed, where we cool our feet in a beautiful stream, while a village dog who has followed us down the mountain snaps at butterflies as big as birds.

Lunch is in Ban Doong Village, home to an extended ethnic Vietnamese clan of 37 who live with no electricity and – even rarer in Vietnam – no internet. Back on the trail, Vuong stops to show us a poison ivy plant. It looks so innocuous: large, luscious, glossy green leaves with a pale, slightly hairy underbelly. I attempt to burn the image of the plant onto my brain and we press on, eyes darting warily from either side of the path.

A lone hiker greets us on a reedy sandbank, going the opposite direction. It's Ho Khanh, the local man and guide who, as a young man in 1990, first stumbled on the world's largest cave, Hang Son Doong (Mountain River Cave). Through a translator he explains how he knew the cave was "different" as soon as he saw it: "It was very difficult to see the cave because there was so much fog. There was also a very strong wind coming out."

It wasn't until 2008 that Khanh found the cave again and, a year later, Howard and Deb Limbert were among the first expedition to go inside and map it. At five kilometres long, 200 metres tall and 150 metres wide it could swallow any other cave in the world; inside, 80-metre stalagmites vie for space with a thriving forest boasting trees up to 30 metres tall. It's so big it generates its own weather, with clouds billowing out like smoke from a giant dragon. In 2013 Oxalis was granted exclusive permission to take tours into Son Doong. The five-day trek costs $US3000 and spaces are limited to 500 each year.

After bidding Ho Khanh goodbye and many more river crossings, we eventually arrive at Hang En's side entrance. Donning hard hats fitted with lamps we press on into the darkness, scrambling up a sandy incline to a path strewn with giant boulders. At the top of these we stop, speechless. Hang En's huge, central chamber, with its turquoise lake and sandy underground beach, opens out before us. It's stunning.

Later that evening we feast on pork, chicken and rice prepared by the Oxalis team of 15 staff, toasting our fortune with plastic mugs of rice wine. One by one, we head to our tents for sleep.

The next day, after an early-morning dip in the cool, deep waters of the lake, our guides lead us further into the cave. We stop to admire strange formations of calcium and 400-million-year-old fossils. Above us, thousands of unseen but noisy bats cling to the cave roof, creating a humid funk that reminds me of visiting the zoo on a wet day.

Any discomfort is forgotten, though, when we emerge to find the cave's stunning main opening. It features an impossibly high roof that frames a view to a distant forest of green, like some portal to another world. I sit and contemplate the beauty, recording the moment with my camera and willing it to imprint on my memory, forever.

Sadly, too soon, it is time to say goodbye to our fantasy world and we each grit our teeth for the next task at hand: retracing our steps, back over 11 kilometres, and 30-odd river crossings, past the dreaded poison ivy, through clouds of black-and-azure-winged butterflies and now up the vicious 45-degree incline to our waiting bus home.

When I finally arrive there I sip a cold, quenching beer, contemplating what has been one of the most beautiful and fascinating experiences in my life. Soon, though, thoughts turn to saving money for my next trip: a visit to Hang En's big brother, Hang Son Doong.

TRIP NOTES

MORE INFORMATION

oxalis.com.vn

GETTING THERE

Several airlines fly from Melbourne and Sydney to Ho Chi Minh City, Hanoi and Da Nang, in Vietnam. See virginaustralia.com, thaiairways.com, vietnamairlines.com, qantas.com.au, virginaustralia.com, jetstar.com and cathaypacific.com.au. Cave expeditions depart from Phong Nha, a small village about 45 minutes' drive from the nearest town, Dong Hoi City. Most travellers arrive via bus or train from Da Nang, Hue or Hanoi but it is possible to fly from Ho Chi Minh City with vietjetair.com to via Dong Hoi. For a comprehensive arrival and departure guide, see oxalis.com.vn/arrival-departure-guide.

STAYING THERE

Cheap backpacker and homestay accommodation is plentiful in Phong Nha. We chose a friendly, three-bungalow place overlooking rice paddies and mountains called Sy's Homestay. Rooms cost from VND 800,000 Vietnamese dong a night, including breakfast.  See sy-homestay.co.

For something more sophisticated, Oxalis's Chay Lap Farmstay has several accommodation types, including standard rooms starting from about VND 800,000, if you book online, in advance.  See chaylapfarmstay.com. For more accommodation options see oxalis.com.vn/phong-nha-accommodation-guide.

SEE + DO

Oxalis provides a cave expedition packages for a range of duration and physical abilities, from one-day tours to Hang Tien for VND 2 million to two-day/one-night treks to Hang En VND 6.5 million and immersive five-day-four-night experiences to Son Doong, VND 66 million (bookings limited to 500 people a year). Or, find your own way to Paradise Cave (entry VND 250,000) and then enjoy splashing around in the fast-flowing waters of Nuoc Mooc Ecotrail (entry VND 80,000, Huong Trach, Bo Tach, on the Ho Chi Minh Road, just north of Paradise Cave).

DINING THERE

Food in Phong Nha is nothing to write home about but there are several decent restaurants, bars and cafes on the main strip. One of the best is Phong Nha Bamboo Cafe, which does Vietnamese dishes as well as burgers. Son Trach, Bo Trach, Quang Binh, Vietnam.  See phong-nha-bamboo-cafe.com.

Peter Barrett travelled at his own expense.

See also: The world's top 10 most stunning underground attractions

See also: On board Vietnam's epic, 1726km slow-speed train

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