We were on the train to Đồng Hới when I first read about the tigers. I secretly thought I could outwit the snakes. But with tigers, I wasn’t so sure.

We had caught the train from Huế to go to Phong Nha-Kẻ Bàng National Park, an area in central Vietnam which borders Laos. The national park is home to the world’s largest cave (a five-day round trek through the jungle), and the third largest, which was only a two-day trek. Perfect, we thought; what could be better than a lovely relaxing two-day trek through a humid jungle and an overnight stay in a massive cold dark cave?

We arrived in the small town of Cuo Lac and checked in for what would be our last comfortable night’s sleep, at Ho Khanh’s Homestay. Ho Khanh had the dubious honour of being the chap who ‘discovered’ the cave, back in the 1970s. What he was doing that far from home, digging around in the side of mountains, I’m not sure. But in 2006, he led the British Cave Expedition to explore the cave, by that time known to locals as Hang En, or ‘Swift Cave’. It was the first time that the cave had been seen by international experts, and was soon declared the third largest in the world.

The first sign we were dramatically out of our depth was the ‘safety briefing’ before the trek. We had walked far longer distances in the past, but the warnings of leeches, snakes, poison ivy and heatstroke made us feel slightly under-prepared.

The night before the trek I had stumbled across a packing list in my emails which had been sent a few weeks before, but I had completely overlooked. After a rushed trip to the clothes market in Huế, I was now wearing a counterfeit Hollister shirt and Fila cap, while Nicola opted for a blouse and leggings. At the safety briefing we met a handful of other people who were joining our trek. They had all the gear, were armed with hiking equipment, energy sachets, and looked fully prepared to take on the jungle. We looked like we were popping out for a pub lunch.

At 10am, a minibus took our group for an hour or so into the jungle, the road winding around the mountains as we climbed. After a final kit check with our trek guides, we entered the jungle. A narrow patch had been hacked through the undergrowth, just wide enough for us to push through with our bags on. The road already seemed very far away as we descended into the valley, clinging to vines to steady ourselves on the slope.

It was already 35 °c and intensely humid; the sort of heat that makes you tight of breath and quickly sticks your hair to your forehead. A few minutes in, and I had given up peeling my sodden Hollister shirt from my back.

After a couple of hours slipping, sliding and stumbling down through the rainforest, we reached the bottom of the slope and found ourselves alongside by a river. Our small group stood at the bank, looking left and right for the onward route. Only when our guide waded in to the water did we realise that the route was the river itself. With no other option, we hoiked up our rucksacks, tucked our trousers into our socks and walked straight into the water. At least it cooled us down.

We walked along the river for a good few hours, dipping in and out as it rose and fell in depth. In some places, it was ankle-deep; in others, it was faster flowing and neared our waists, so we leaned into the current and clung on to each other to stay upright. When we needed water, our guides siphoned it from the river through a purifier. When we needed the toilet, we found a bush and hoped no snakes would emerge. I spent a significant amount of the day scanning the surroundings for tigers, hoping that the others in the group looked tastier than me.

After hours walking along the river, it curved around a huge limestone cliff, and at the end of the next valley, we finally saw the entrance to Hang En cave. When we eventually arrived, it turned out much of the roof had caved in. So, we donned our hard hats, switched on our head torches and scrambled up the rock fall into the cave. As we climbed, the light from the cave entrance faded, and we were left with just our torch beams creating swirling cones of illuminated dust in the blackness.

We climbed carefully, scrambling up boulders, looking for hand and footholds, trying not to think about the fact that we were climbing into a pitch-black cave a day’s walk away from civilisation. When we reached the top of the rock fall, our eyes began to adjust to our new surroundings and we spotted tents pitched on a beach far below, dimly lit by a shaft of light from a hole in the roof of the cave. A final scramble down the other side of the rock fall, a paddle on a raft across a small lake, and we would be at our bed for the night.

It was a relief to finally be in the cave, as it was a lot cooler than the jungle. A vast cathedral of rock, Hang En is over 100m high, and almost 200m wide. You could fit the whole of St Paul’s Cathedral inside, and still have room to spare. It’s absolutely massive. The cave is home to plenty of bats, and almost one million swifts, who flit in and out of the distant entrance as dusk sets in. The only source of light other than torches and fire is the opening at the far end of the cave roof, where by now the late evening light silhouetted distant trees and vines against the darkening sky.

In true Vietnamese style, and despite that fact that we were in a cave, the guides had brought plenty of food, and it was all excellent. They cooked better food in a cave in the middle of a jungle than I could in a fully stocked kitchen.

At night, every sound echoes around the cave, ricocheting off the walls and ceiling. Nicola and I lay still in our tent, a thin layer of canvas between us and the bats, swifts and thousands of tonnes of rock over our head. A distant rumble deep into the cave could have been a rock fall, a thunderstorm, or wild animals. And we were two tiny, powerless humans wondering why on earth we weren’t in a hotel. We didn’t sleep well that night.

After a long, restless night, we had the return trek to Cuo Lac to look forward to. It was, we were promised, more difficult than the first day. We started by retracing our soggy footsteps through the river, then followed our guides as they cut into the jungle once more. But this forested mountain was steeper and hotter that the previous day. We climbed over fallen branches, clinging onto vines to hoist ourselves up the slope. The closest thing I can think of is that it was like climbing an escalator. But an escalator where the steps have been smoothed into a slope, covered in slippery earth, crisscrossed with polished tree roots, and the temperature turned up to 40°c. Oh, and the escalator in question is at least a few times as high as the Shard. It was horrendous.

We had been climbing for a while when Nicola asked one of the guides how far it was until the peak. He told us we were barely a quarter of the way through the climb, and it would be a while before the path evened out. My head had started pounding, and despite drinking litre after litre of water throughout the climb, I think I had become dehydrated. Eventually my head was throbbing to the point where I could barely stand up, so I sat, head in hands, downing little rehydration sachets to try and muster some energy. To add to the mild sense of peril, we had to catch a night train in a few hours. If we missed it, there were no spare seats on the train until after the weekend, and we would be stuck in Cuo Lac for a few days. It wasn’t looking good.

I wasn’t quite sure how we were going to get out of the jungle, and sat down on a tree root, feet dug into the slope to stop myself from slipping, Nicola sitting alongside me. Then, we felt a few drops of water from above. At first I wasn’t sure if it was rain or sweat. Exhausted, we sat with our palms facing skywards, squinting up into the forest canopy to see where the droplets were coming from. Then, with a crack of thunder, it began to fall heavily. Proper rain. Big drops, which splashed down, bouncing from leaves, drenching us, cooling us. Nicola and I looked at each other and rejoiced. Further down the slope, we heard the others in our group whooping and cheering.

With my headache receding as quickly as it had started, the cool rain gave us a new-found energy, and the determination to continue to hack our way through the undergrowth back to the awaiting minibus.

But the even the rain provided one final hurdle. Leeches. As the water streamed off leaves and branches, with it came leeches, looking for blood to feed on. Not content with sliding around on the edge of a mountain, making sure we weren’t grabbing onto snakes instead of vines, and keeping an eye out for tigers, we now had blood sucking creatures falling from the sky. At this stage, we really didn’t care, and batted them away as they fell. By this point, we knew there were just a couple of hours of jungle before we reached the road.

When we eventually emerged out of the trees onto the asphalt, our trusty minibus was waiting, two headlights cutting through the torrential rain. I had never been so happy to see a van.

We made it to Đồng Hới train station with half an hour to spare, and boarded our sleeper train at 9pm. There we sat in the flickering darkness of our bunks, whispering to each other and watching shadows pass the window as the train crawled out of Dong Hoi station into the Vietnamese night.

Despite the flimsy bunk beds, the locals snoring above us, and the clattering of the train, we had never slept so soundly.

We stayed at: Erm. A tent. In a cave? (oxalis.com.vn)
We also stayed at: Ho Khanh’s Homestay (phong-nha-homestay.com)
We ate at: The same cave. It’s a long way to go for a meal. Don’t.