There, we donned our hard hats and headlamps and silently entered single file, darkness enveloping us, just the light of our flashlights illuminating our path. A few hundred feet in, we reached a mountain of boulders. As we scrambled up, the light became more intense as we gained height. On reaching the summit we were stopped dead in our tracks by the view before us — the cave’s gigantic main cavern.

At 300 feet in height and 600 feet across, the cavern is big enough to fit a Boeing 747 with room to spare. The space was flooded with rays of natural light coming in from an arch high above us. The beams of light illuminated a yellow sand beach hundreds of feet below, surrounding a calm turquoise pool.

A team of porters who had gone ahead of us were already down on the beach, some pitching our tents for the night, others keeping a fire burning ready to cook dinner. As flames flickered, catching on the light breeze being drawn through the cave, smoke swirled upward in the musky, dank air.

Two nights before I had been having dinner with Howard Limbert, a gregarious 57-year-old cave expert who left his job in England as a biomedical scientist in 2012 to devote his life to exploring the caves of Vietnam. Mr. Limbert spoke with infectious enthusiasm about his many research expeditions during the 1990s to map and measure Hang En and the other caves in the region, using laser technology. He told me, between deep drags on a Vietnamese-brand cigarette, that by trekking to Hang En, I would have one of the most exciting adventures of my life. As I stood looking out over the vast, otherworldly space before me and reflecting on the hike to reach it, his words rang true

In this jungle region of central Vietnam, the Hang En cave is one in a series of mind-blowing caverns discovered by Mr. Limbert, his wife, Deb, 54, also a caver, and their colleagues from the British Cave Research Association. The most enormous cave here is Son Doong Cave, which ranks — alongside Miao Room in China and Sarawak Chamber and Deer Cave in Borneo — as one of the world’s largest caves. Son Doong last year for the first time became accessible to a limited number of tourists, thanks to Oxalis, the most established and reputable company running tours in the region’s caves.

In Son Doong’s vast caverns, forests of 100-foot-tall trees thrive in spaces big enough to accommodate 40-story skyscrapers. Colossal 260-foot stalactites, not to mention monkeys, hornbills and flying foxes, are also found in Son Doong’s surreal habitat, first fully explored in 2009. However, Son Doong, at $3,000 for a six-day trek into the deep innards of the cave, was far out of my price range and, in any case, sold out. Only 250 total spots were available for 2014, but slightly more will be offered in 2015, with bookings beginning in November.

However, other remarkable caves are nearby and much more affordable. There is the Hang Ken cave with its waterfalls spilling into large lagoons, shimmering goldlike mineral deposits and soaring columns created over thousands of years when stalactites hanging from the cave roof met stalagmite formations building from the cave floor. I decided on the larger and more dramatic Hang En cave on Mr. Limbert’s strong recommendation as it offered the chance to camp inside the colossal main cavern, and would be a wildly adventurous journey.

The Hang En tour involves a difficult daylong trek over limestone mountain paths and along riverbeds to the remote indigenous village of Ban Doong. As a Hanoi-based travel journalist and photographer who has lived in the region for nearly four years, I relished the idea of discovering something fresh and different. And as a competitive cyclist and mountain runner, I liked the muscle-burning aspect.

The expedition began in Phong Nha, a sleepy community of 1,000 people about six miles from the start of our trek. The town has acted as a base for cave-visiting for the past few years, with numbers swelling after the 2011 opening of the easily accessible Paradise Cave, popular with Vietnamese tourists as it can be reached by boat with no trekking required. Before the cave tourists arrived, the town was poor, with income mainly derived from farming, fishing and hunting.

The Paradise Cave and the more adventurous cave expeditions have turned this former tourist backwater, which lies 12 hours by train from Hanoi, into a bucket-list destination for intrepid travelers, with a choice of three caving adventures now on offer via Oxalis Adventure Tours: the Son Doong cave trek; treks to the Tu Lan river cave system that includes the Hang Ken cave at a cost of $260; and the Hang En trek that I had opted for, at $275.

Mr. Limbert and I had met for dinner at his favorite Phong Nha restaurant, Quan An Vung Hue. Sparsely decorated, it is hidden behind the small town’s narrow main street. Soon, an array of his culinary recommendations lay before us — a bowl overflowing with marinated, grilled ribs, a plate of crunchy greens sautéed with garlic, a dish of beef stir-fried with pineapple, and the obligatory mound of rice. Grazing on our feast, Mr. Limbert recalled the four full days it had taken him in the early 1990s to complete what had been for me only an overnight journey from Hanoi.

Clearly in his element, he leafed through photographs as he recalled those early days. In one, his wife, Deb, stands on a rutted stone track, cradling a rusted machine gun in her hands. “That was in 1990,” he said. “You almost couldn’t put your foot down without treading on remnants of war. There were guns, bullet shells and bomb casings everywhere.”

After dinner, we cycled toward my guesthouse along pitch-black streets devoid of streetlights. The chatter of families emanated from the small, one-story homes on the river’s edge, some made of concrete, others little more than shacks, in which several generations live crammed together. Otherwise, the only noise was the cacophony of buzzing and whirring insects from the riverbank. As we rode, a small boy appeared from nowhere and hitched a ride on my bike’s rear rack. “That’s normal here — bicycles are like a free informal taxi service,” Mr. Limbert said, smiling.

We arrived at Ho Khanh’s Homestay, owned by Mr. Ho Khanh himself, now a local celebrity who is credited with discovering the Son Doong entrance in 1990 while on a hunting trip. He later led Mr. Limbert and other members of the initial cave exploration team to Son Doong. Never afraid of a superlative, it was then that Mr. Limbert made his bold claim, bidding me the good-night promise that the next day I would be doing one of the “best, most amazing things it’s possible to do in Vietnam” — the Hang En trek.

The next morning I arrived at the Oxalis Adventure Tours office and met my fellow cavers — seven Australian men and women in their mid-20s, some on vacation, some on extended backpacking trips. After being kitted out in Cambodian army boots and given waterproof backpacks and water bottles, we set off by van for the Phong Nha-Ke Bang National Park, a few miles from town.

The road passed through a national park gate where the vehicle was inspected — tourists can travel in without a guide, but trekking off the road without one is prohibited. The road then swiftly carved a vertiginous route skyward, hairpins twisting around the thickly forested hillside to the trek’s start point. There, we got out of the van and prepared for 1,500 feet of vertical descent down a dense jungle path. Led by our boyish guide, Hoang Thai Binh, a lean and athletic 26-year-old, we dropped into single-file formation, the sound of everyone’s increasingly heavy breathing combining with the sound of insects buzzing and twigs snapping under heavy footsteps. Picking a route down the muddy, rocky path, some struggled to stay upright, grabbing hold of the long grasses and tree branches that crowded in on the narrow, snaking path.

On reaching the valley floor we stopped at a stream in the shade of a magnificent tree canopy. We took seats on boulders while our porters dropped their weighty makeshift backpacks made of recycled rice sacks. Dripping with sweat from exertion, we all eagerly sucked down our water, as the porters pumped water from the stream to replenish empty bottles. “You thought that was hard?” asked our guide, Binh, in excellent English. “We go back out the same way. Just wait ‘til you try climbing it.”

Once we all caught our breath and quenched our thirst, a much more sedate hike began through the jungle following the meandering of the shallow Rao Thuong River toward Hang En. Swarms of butterflies wove a dance along the path in front of us and flirted with the water’s surface. The area is blessed with myriad flora and fauna species, both inside and outside the caves. Tigers roam in more remote areas, while the silhouettes of flying foxes can often be seen gliding through the air at dusk. The rare Hatinh langur (a small primate species) is another star attraction. Found only here and in the neighboring jungle of Laos, it is seldom spotted by tourists, but the guides, with their keen eye for wildlife, report frequent sightings.

After an hour we emerged from the jungle into a clearing where the remote Ban Doong village stands, its stilt houses interspersed with plots of fruit and vegetables and surrounded by soaring hills carpeted by trees rising from the valley floor. The village is likely to remain isolated for some time, as the only way to reach this tiny settlement of about 40 Bru-Van Kieu ethnic minority people is how we had arrived — on foot.

The village chief had been out in the jungle for days, but his wife was home. She welcomed us warmly as a gaggle of children played at her feet. Walking outside, she began to address us, via Binh’s translation: Of the dozens of ethnic groups in Vietnam, “we are the poorest!” she exclaimed, nonetheless smiling broadly. In reply, and in a classic demonstration of Vietnamese frankness, Binh bluntly put it to her that perhaps they ought to move somewhere offering more forgiving conditions. At this, her smile vanished. “Here we have land!” she countered, jabbing her finger at the dry terra-cotta earth. “We can’t farm rice, but we can farm corn, fruits and we can fish, we can hunt. Where would we go? What would we have if we moved?” Then, as abruptly as her temper frayed, her easy grace returned as she swept a crawling child into her arms and took a seat back in the shade.

As we pushed on, the valley opened up farther — magnificent, lime green hills rising all around us. A couple of miles later, one of the mighty arches of Hang En cave came into view.

After forging deep into the cave, still following the Rao Thuong River and clambering up to the lookout point with its views over the beach campsite, we headed down to the tents where we dropped our gear. The river we had followed carves right through the cave for over a mile, finally emerging in another valley on the far side of the limestone massif. We followed the water, first into darkness and onward into gradually strengthening light that built to reveal another mountain of rocks towering over us. Again we climbed, this time reaching a vantage point that gave us our first sighting of the most colossal of Hang En’s arches.

At almost 400 feet high, its scale is spectacular — it’s so large it could almost accommodate the Statue of Liberty. Looking down at the floor beneath it, we watched as young boys scurried back and forth. “They are picking up infant swifts to take home and cook and eat,” Binh said. “The recently hatched birds either attempt to fly and can’t, or they simply fall from the nests.” At this time of year the birds are easy prey, but when they aren’t falling, the boys scale the arch using ropes to fetch them directly from the nests. When we reached the bottom they proudly showed us their day’s haul, the swifts clinging to their T-shirts.

Ravenous, we made our way back to the camp, our bright orange tents now illuminated by lamps as dusk fell outside. We shed our sodden clothes and took a dip in the warm water of the turquoise pool as the porters finished preparing dinner.

Having freshened up, we sat down on a mat to find skewers of tender flame-cooked pork, sautéed beef and a local specialty of lime leaf dip laid out. Before eating, glasses were filled and a toast raised. The local ruou rice wine had been described by Mr. Limbert as the biggest danger to those entering the caves, as the porters have a highly sociable nature and an equally highly persuasive way of offering “just one more” glass.

Satiated and intoxicated to various degrees, one by one we peeled off to our tents, which we left open in the hope of catching sight of a flying fox gliding across the arch high above us.

The next morning, following a breakfast of noodles, spring rolls and fried eggs, we trekked out of the cave the way we had entered and through the long grass back to Ban Doong village. The chief had arrived home with, among other things, a 1.5-liter-water bottle of wild honey. Freshly extracted from a hive, its amber color and unusual sweetness led me to open negotiations. On agreeing on a price, his wife’s mile-wide grin told me I’d paid well over the odds for the pleasure of adding another three pounds to my already weighty pack. However, Binh assured me that the amount of money I paid was sure to come in very useful with a wedding in the village fast approaching.

Later that evening, back in Phong Nha, my travel companions and I reflected on the two-day journey at a roadside food spot, which the owners recently had renamed on a rustic wooden sign, “The Best BBQ Pork Shop in the World ... Probably.” We deemed our meal the second best BBQ pork, having been narrowly beaten by the pork prepared over the previous night’s campfire in the Hang En cavern. Over dinner, we all heaped praise on the cave trek, with everyone ranking it above the standard Vietnam trips to the northern mountain town of Sapa and the Unesco Natural Heritage site of Halong Bay.

As I savored the succulent pork and the good company, I could not help but think that the lure of the big caves will make a tourism boom almost inevitable in this distant corner of Vietnam. The Limberts think so too, which is why, along with the Oxalis tour company, they are developing other caving areas to ensure that none are swamped. The Hang En and Son Doong caves are inside the Phong Nha-Ke Bang National Park, a Unesco site, a designation that should give the area a measure of protection from development. (It’s not that developers haven’t eyed it for major projects, with one proposed plan to run a cable car to the mouth of the Son Doong.)

Mr. Limbert is wary but upbeat: “It’s not easy and there are challenges all the time, but when you get out there, when you go remote, it’s obvious this is a very, very unique place and well worth fighting for.”

By David W. Lloyd - The New York Times - October 31, 2014