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Picture yourself waking up on a sandy beach beside a turquoise underground lake, illuminated by sunlight pouring in through a giant skylight. Imagine finding yourself inside a natural cathedral made of sinuous stone, shaped by millions of years of erosion. Visualize yourself fording a subterranean river, and being stopped in your tracks by the sight of mist and clouds forming on the inside of the cave so vast that the brain struggles to comprehend its exact dimensions.

Every year, a few lucky visitors to Vietnam get to enjoy this very experience. The world’s largest cave, Hang Son Doong (Mountain River Cave) was discovered in the dense forests of Vietnam’s Phong Nha-Ke Bang National Park in 1990 by a local man, Ho Khanh, as he was seeking shelter from a storm. The cave then lay forgotten until Khanh led British caver Howard Limbert and his team there in 2009 to map Hang Son Doong for the first time.

Hang Son Doong is vast. It’s so big that you can fit any of the world’s other largest caves comfortably inside it, as well as several 40-story skyscrapers. The cavern reaches 503m (1650ft) in height and 175m (574ft) in width and the cave system is 9.4km (5.8 miles) long, winding beneath karst mountains. Indeed, the cave is even larger than previously thought – in 2019, British divers explored Hang Son Doong’s underground waterways and discovered a tunnel linking the cave to Hang Thung, another huge cavern.

How can I visit Hang Son Doong?

The world's biggest cavern can only be visited on pricey organized caving tours, and there are strict limits on the number of visitors admitted every year. But if you can't stretch to the cost or time commitment of a Hang Son Doong expedition, surrounding Phong Nha-Ke Bang National Park offers plenty of other cave-based adventures.

Covering 1233 sq km (476 sq miles), this UNESCO World Heritage Site bristles with some of the oldest karst mountains in Asia, riddled with extraordinary cave systems, which are the park’s top draws. Above-ground attractions include excellent jungle trekking, excursions into the picturesque countryside and sites linked to the American War – it's certainly one of the highlights of a trip to Vietnam.

How do I book a Hang Son Doong expedition?

Howard Limbert helped to establish the adventure tour agency in 2011, and in 2014 the company launched multi-day exploration tours of Hang Son Doong. Groups set up camp in the spectacular Hang En cave en route, taking a swim in its subterranean lake before descending into Hang Son Doong using safety harnesses.

You'll spend two days exploring giant passages with vast rock formations, bathing in underground pools and streams, gazing upon areas of primeval forest growing inside the cave, and sleeping in scenic campsites beneath two dolines (giant sinkholes) that fill the cave with effervescent light. Wildlife sightings are common, from monkeys, flying foxes, snakes, bats and birds to endemic species of fish, shrimp and creepy crawlies that evolved to exist in complete darkness.

Access to the cave is limited to 1000 visitors per year, so demand for places is high. Each group of ten is led by at least one of the British cavers who took part in the original Hang Son Doong expedition, and bookings open for the caving season (January to late August) up to two years in advance. Realistically, you can expect to join a tour around 18 months from the date you book.

The four-day/three-night expedition costs US$3000 per person (of which US$620 goes towards the upkeep of Phong Nha-Ke Bang National Park). This includes a pick-up and drop-off at the city of Dong Hoi (the nearest town with an airport) and the services of a cave expert, tour guide, safety assistants, cooks and porters, as well as food and drink, accommodation in tents, and all necessary safety equipment. However, you'll need your own comprehensive travel insurance.

Are other caves in Phong Nha-Ke Bang National Park worth exploring?

If you can't join a Hang Son Doong expedition, consider joining an overnight trek with Oxalis to Hang En, another of the world's largest caves. You'll camp on a sandy beach beside an emerald pool inside the cave and trek through the primeval jungle to get here.

Oxalis also offers expeditions to the Tu Lan cave complex. Choose between overnight camping trips or multi-day expeditions that include abseiling into underground rivers, exploring Hang Tien, technical training in the darkest part of the cave and encounters with Nguon people in a remote mountain village.

Oxalis also offers a multi-day , hiking through primary forest in search of wildlife such as gibbons, wildcats, langurs and wild boar, and exploring six caves that have only recently become open to guided tours.

Another operator is , with various overnight and multi-day trips, including a three-day tour of Hang Thung, the giant cave linked to Hang Son Doong. Highlights include rappelling and camping by the Ma Da subterranean lake. There's also an overnight trip to Hang Pygmy – the world’s fourth largest cavern – with paddleboarding on yet another spectacular subterranean lake.

What if I don't have time for an overnight trip?

Time too tight for an overnight trip? From the village of Son Trach, you can day trip to Hang Tien, the largest of the dry caves in the Tu Lan system, where you can view astonishing calcite formations and rimstone pools. Alternatively, you can whizz above Hang Toi (Dark Cave) on a 400m-long (1312ft) zipline and squelch through thigh-deep mud in pitch darkness before finishing off with a kayaking session and a swim.

Visits to Phong Nha Cave involve a leisurely boat trip from Son Trach, passing rice paddies and limestone peaks before your craft glides silently past immense illuminated stalactites and stalagmites. If you climb the 330 steps to Tien Son Cave, you can view 9th-century inscriptions from the Cham era.

Even if caves are not generally your thing, Paradise Cave may change your mind. Electric buggies ferry visitors to this striking cavern surrounded by forested karst peaks. The small size of the cave entrance belies the staggering spectacle of the colossal cavern inside, flanked by otherworldly rock formations. Both Phong Nha Cave and Paradise Cave get busy with tour groups, so it’s best to visit first thing in the morning.

How should I choose a tour operator?

Phong Nha-Ke Bang National Park is peppered with unexploded ordnance from the American War, and many areas can only be visited on foot with a licensed tour operator. Paradise Cave and Phong Nha Cave are accessible on day trips, but to see Hang Son Doong, Tu Lan, Hang En and some other caves, you'll need to join an overnight trek with a licensed guide.

Oxalis is the exclusive operator for , and also runs multi-day jungle expeditions to Hang Ba, while Jungle Boss has exclusive rights to overnight treks to Hang Pygmy and three-day, two-night explorations of Tiger Cave. They also offer multi-day jungle treks and rappelling adventures to the base of Do Quyen waterfall. , led by knowledgeable jungle guide and conservationist Hai, arranges visits to the park's Wildlife Rescue Centre and daytime and nighttime treks in the jungle.

When choosing an operator, ask about group sizes and the operator's guide-to-client ratio, and confirm what exactly is included in the price. Many places to stay in Son Trach or Dong Hoi can arrange jungle treks inside Phong Nha-Ke Bang National Park with independent local guides, but quality varies, so it’s worth asking other travelers for recommendations.

When should I go caving in Phong Nha-Ke Bang?

As with other parts of Vietnam, the best time to visit varies depending on what you plan to do and see. Winters (December to March) can be crisp and cold, with an equal number of sunny and rainy days, but Hang Son Doong has its own microclimate, and January and February are the best times for watching sunbeams penetrate deep inside the cave.

In late January or early February, all of Vietnam goes on vacation for the Tet festival, so it’s the busiest time of year to travel. Phong Nga residents celebrate Tet in a raucous way, sometimes using unexploded ordnance in place of fireworks and sharing jugs of rice wine with passers-by.

April to May is arguably the loveliest time to explore the national park; the weather warms up, the landscape turns a lush green and wildflowers are everywhere. You may also witness the phenomenon of clouds and mist forming inside Hang Son Doong.

Summer (June to August) is the hottest, most humid time of year, but the trekking trails to Hang Son Doong and other caves are shaded by tree cover. Inside Hang Son Doong, the temperature hovers at a balmy 22–25°C (72–77°F), and it’s warm enough to swim in the streams and underground lakes inside the cave. Autumn (September to November) brings tropical storms and heavy rain, with flooding in the jungle and inside river caves, so caving trips only run from January till late August.

How much time should I spend here?

If you’re taking part in a Hang Son Doong expedition, you need to set aside six days, but if you’re looking to just dabble in cave exploration and take in Phong Nha-Ke Bang’s above-ground attractions, you can pack a lot into four or five days. Consider an overnight trek to Hang En cave or the Tu Lan cave system, then take a day to explore the beautiful Bong Lai Valley by bicycle, followed by day trips to the Phong Nha Cave and Paradise Cave. Or perhaps combine an overnighter to Hang Pygmy with day jaunts to Hang Tien, Hang Toi, and the Bong Lai Valley.

Is it easy to get in and around Phong Nha-Ke Bang National Park?

Phong Nha-Ke Bang National Park spreads west to the border between Vietnam and Laos. It’s centered on Son Trach (also known as Phong Nha Village), a stop for most open-tour sleeper buses from Hanoi and Hoi An. Dong Hoi, 50km southeast of Son Trach, is the nearest city with a domestic airport, while the nearest international airports are in Danang and Hanoi.

Dong Hoi is also a stop on the train route between Hanoi and Ho Chi Minh City, with frequent public buses connecting Dong Hoi and Son Trach. Accommodations in Son Trach also organize private and shared car transfers to/from Dong Hoi, and you can also get here using motorcycle transfer services from Hanoi, Hue and Hoi An.

Exploring the more accessible parts of Phong Nha-Ke Bang National Park is best done with your own wheels. Cycling or scootering is a wonderful way to traverse the rural roads on the fringes of the park; bicycle and scooter rental is available from guesthouses, homestays and hotels in Son Trach, and staff can sometimes provide maps. River taxis connect Son Trach with the beautiful, unspoiled Bon Lai Valley, and bicycles and motorbikes can be taken on board.

The condition of roads inside the park varies and routes are not well-signposted, though you can download route information from the independent website . Consider joining a motorcycling tour with , with a knowledgeable, enthusiastic local team helping you discover the park’s highlights. They also arrange motorcycle transfers to destinations further afield, such as Hue or Khe Sanh.

Frequently asked questions

Do I need prior caving experience?

No prior caving experience is necessary; the expedition agency will take care of everything.

What gear should I bring?

Essentials include quick drying, long-sleeved shirts and trekking pants, breathable trekking shoes with good grip (rather than waterproof/Gore-Tex boots), warmer layers for winter months, and swimwear, shorts and T-shirts for summer. Other must-carry items include mosquito repellent, sun cream, antifungal cream, environmentally friendly personal hygiene items and a drybag and powerbanks for your electronic gear.

How physically fit do I need to be?

Some trekking experience is essential. In the year before a Hang Son Doong expedition, aim to complete at least one overnight trek and one or two day hikes of at least 8km that involve steep mountainous terrain. Take regular exercise to maintain an adequate level of fitness, as the Hang Son Doong expedition involves trekking on rocky terrain, some steep climbing, abseiling inside the cave, fording rivers and scrambling over large rocks.

Are there age restrictions on caving expeditions?

Most multi-day caving and jungle trek trips are open to participants aged 16 (or 18) to 70. However, Oxalis also offers day-long introductions to the Tu Lan cave for kids aged 6 and older.

What’s the accommodation like?

On overnight cave expeditions and jungle treks, you'll sleep in tents, with drinks and meals provided by the operator. Camping equipment is transported to the campsites by porters. Many cave expeditions camp on sandy beaches beside underground rivers and lakes, providing opportunities for swimming.

What is the bathroom situation like?

Campsites inside the caves have eco-friendly composting toilets. There are no showers, but you can wash in cave rivers and lakes. Shampoo or soap should not be used, as it would contaminate the water; for some campsites, you’ll need dry shampoo and wet wipes.

Do I need to bring money on the trek?

There are few opportunities to spend money while you're trekking, but infrastructure is limited in Son Trach, the village closest to the caves. There's a single ATM, which is occasionally out of order, so bring plenty of cash to cover spending before and after your expedition.

Should I tip?

While tipping is not deeply ingrained in Vietnamese culture, it’s always appreciated. A standard gratuity for tour guides is around VND 100,000–200,000 per day (less for porters and cooks), depending on the length and complexity of the tour. Tip in the local currency, ideally in small denomination notes.