A night inside the world's largest cave network in Vietnam

Emma Pearson treks through jungle, abseils down cliffs and shimmies through crevices to spend the night in one of the largest caves in the world

Inside Pygmy Cave. Courtesy Jungle Boss
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As far as hotel rooms go, I've tried the lot. Five-star resorts, one-star regrets, tree houses, wooden huts, caravans, riverboats and even, in dire times, the odd airport floor. However, my current place of rest is by far the most impressive – in terms of both originality and size.

I spend the evening sleeping inside the world's biggest cave network, waking up on a sandy hillside next to a river inside the Pygmy cave within Vietnam's Phong Nha National Park. The cavern is a colossal 100 metres high, the size of London's Big Ben, and 845 metres wide – longer than Burj Khalifa is tall.

But getting to my home for the night – the world’s fourth-largest cave – is no walk in the park; it ­involves a 20-kilometre round-trek through the jungle, an abseil down cliffs, a swim through dark-­water rivers and a shimmy through ­terrifying crevices.

Hang Pygmy opened to the public only two years ago and guides from tour operator Jungle Boss need government approval to take visitors there (meaning there's no risk of the network being diluted by a barrage of copycat guides). They provide everything ­– headlamps, harnesses, even hot food as we make camp inside the cave – so all I need to do is absorb the most spectacular hotel room I'll ever experience.

I was picked up bright and early by our guide, Bien, who took me to Jungle Boss HQ for a safety briefing before we set off. I was given bug spray, a waterproof backpack, sturdy boots and a waiver to sign.

We then embarked on the one-hour bus ride to Phong Nha National Park. The Unesco-listed site is one of only two natural World Heritage Sites in Vietnam, and the colossal limestone peaks and vibrant green foliage that greeted us looked like they were straight out of a Jurassic Park film. I bade goodbye to civilisation and started my first 8km hike through the jungle. Any hopes of a gentle stroll to the cave were almost immediately dashed, as only a few hundred metres in, we arrived at a waist-deep stream. I was just about to turn back, certain we must have wandered off-course, when Bien plunged forward into the ­oncoming water.

It was a sign of things to come. I gave up counting after crossing the fifth stream and instead focused on clambering over razor-sharp rocks and preventing a small army of leeches from ­invading my clothes.

I trudged down mud, which felt like walking through custard, while my guide strode ahead as if moving along the smoothest of pavements – wearing only sandals, might I add. I learnt that nothing bothers Bien and that, if I was expecting to be spoon-fed, I had another thing coming. You're scared of heights? Don't look down. You've slipped? Get up. Being nibbled alive by flesh-eating leeches? Get over it.

The trek to Pygmy cave. Courtesy Jungle Boss

With more than 14 years’ experience in the jungle, Bien, 34, and his 19-year-old helper, Bom, tottered over fallen tree trunks and craggy rocks completely unfazed. But, while there is no hand-­holding, the Jungle Boss team take safety very seriously indeed, and they don’t mince their words.

"If someone isn't fit enough or they start to struggle, then we take them back," Bien explains, ­solemnly. "I've been leading this tour since it started two years ago and I've lost count of the times people have given up, or we've insisted on taking them back. It's a difficult trek and we can't take any risks." Determined to prove myself worthy, I embarked on the next part of the tour – a 4km cave trek in total darkness that led us to the entrance of Hang Over cave. It is the third-largest cave in the world, with an entrance measuring 120 metres in height and spanning 140m in width.

It was spectacular, and I was rendered speechless as we filed into the darkness with nothing but headlamps to guide us. The daylight behind us dwindled as we made our way into the abyss. The skeleton of a slain water buffalo lay about a kilometre in, a remnant of when tigers prowled these jungles about 30 years ago. The group fell silent – whether in awe or fear is anyone's guess.

Eerily gothic-like limestone formations twisted up and out of the cave floor, while a river flowed rapidly alongside us. The cave was otherworldly, at points looming so low we had to crouch, at others rising so high that we couldn't see the roof. In parts, the terrain was treacherous, alternating between sharp rock and cascading mounds of slippery clay. This is not a tour for the faint-hearted. It's generous to say that a third of the group managed to remain upright, and by the time we reached our final cave we were all filthy (with a few bruised egos and limbs between us). But the best – or worst – was yet to come.

Our final descent into Hang Pygmy involved a 23-metre abseil down, again in absolute pitch-black, scrambling through rock tunnels before ending in a vertical drop. It was enough to shred nerves of steel, but as we passed this final test, we rounded yet another gigantic rock boulder and saw what we had struggled so ­tirelessly for – one of Earth's ­greatest spectacles.

A mesmerising light beamed into the blackness, from a cave opening so vast you could fit a double-decker bus in there (or 21 buses stacked, to be precise). A natural waterfall trickled from the ceiling on to an elevated platform, creating the perfect outdoor shower.

The fading light illuminated the magnificence of the cave. It was a sight to behold, and one that was made all the more breathtaking by what we had endured to get there. Fewer people have made the journey into these caves than have been to the top of Everest, and standing here, covered in mud, I felt like I was on the top of the world.

Emirates and Etihad fly direct to Hanoi International from Dh2,700. From there, take an internal flight to Dong Hoi. For information on this tour, visit jungle-boss.com