As the sun dips below the horizon, I pitch my tent on the sand.
My fellow campers are busy setting up their beds for the night. Leaving my door unzipped, I lay on top of my sleeping bag and look out at the magnificence of our surroundings.
This spot in Vietnamis a strip of sand like no other as it lies deep within Phong Nha-Ke Bang National Park - inside the colossal cave chamber of Son Doong.
Exploring inside feels like you are wandering the heart of the Earth.
Only 1,000 annual visitors are permitted, in groups of 10, so the six-day trek there sells out quickly with the only approved tour operator, Oxalis Adventures.
Laura Sharman recalls pitching her tent on the sandy riverbank of Son Doong cave in Vietnam. The first night of her six-day trek is spent camping on the beach of Hang En cave (file image above)
The cave has been confirmed as the world's largest by Vietnamese officials and Guinness World Records, which in 2012 stated that it is 655ft (200m) tall, 490ft (150m) wide and 'at least' four miles (6.5km) long.
At the time the record was acknowledged, the organisation admitted that the cave could be even bigger, because it had not been fully surveyed.
Oxalix Adventures puts the length closer to 9km (5.5 miles), withenough space for its sprawling jungle, river, and weather system, and a 60-storey skyscraper.
Remarkably, it remained sealed for millions of years before it was discovered by accident in 1990.
It was a local man named Ho Khanh who would stumble upon the subterranean wonder while seeking shelter from a storm, my guidetells me.
The villager had been foraging for an expensive plant known as agarwood and came across a cave entrance below a cliff, but as he got closer he saw mist blowing out to the sound of a roaring river inside.
Almost two decades later, its location was shared with caving experts and the first pilot tours were held in 2013, followed bythe official launch of Son Doong Expedition a year later.
It's the most surreal campsite I have ever encountered.
As I look out of my tent, I take a long deep breath as the sun casts an enchanting glow throughout the cave chamber.
The distant echoes of dripping water provide a soothing lullaby as I hunker down for the night, enveloped by the enormous limestone walls that surround me.
The cave has been confirmed as the world's largest by Vietnamese officials and Guinness World Records, which in 2012 stated that it is 655ft (200m) tall, 490ft (150m) wide and 'at least' four miles (6.5km) long
Son Doong is located deep within Phong Nha-Ke Bang National Park in Vietnam
In the misty morning, the first rays of sunlight pierce through the cave's opening, which reminds us of the dense forest above.
By now, I know it well after the two-day trek to get here with rock scrambling, rope climbs, and ladders.
It had begun with a minibus dropping us off at 9am at the top of a valley and then a descent into the jungle.
Water seeps in through my boots as we wade through shallow rivers and streams. But they provide the grip needed for the steep climb to the first campsite at Hang En cave - the first of two caves on this expedition.
Walking up to its mouth, I could have been fooled that we had reached Son Doong ahead of schedule.
It's the third-largest cave in the world, witha ceiling that reaches 145m (475ft) at its highest point, and the widest section of the passageway reaching up to 200m (656ft), according to Oxaxlis Adventures.
Arriving at Hang En cave, above, Laura says she could have been fooled that her group had reached Son Doong ahead of schedule due to its sheer size
Its sheer scale is impressive, with a spectral atmosphere. The only sound - the chirping of swifts, which could be heard but not seen.
'This cave is named after the birds. They nest here,' our guide said.
There's time for a quick swim in the river while our porters set up the kitchen and the chefs whip up several dishes for dinner.
Then we fuel up for the following day, which involves trekking through the cave, wading through rivers and climbing steep inclines.
I realise how Son Doong stayed hidden for so long as we reach the opening and descend 80m (262ft) underground, using ropes.
The enormous cave passage slowly appears as we lower ourselves down, and the temperature immediately plummets.
After lots of bouldering and scrambling, we arrive at the next campsite - this time within Son Doong's epic cavern.
It's the moment we've all been waiting for and it feels like we are entering another world.
The rocky cavern is so tall, it renders you speechless - particularly when stretching your neck to look up at it.
Light streams through the door to the chamber, highlighting the bright green foliage beyond.
But it's not enough to heat the sand on the ground, which is cold between my toes.
The Son Doong cave remained sealed for millions of years before it was discovered by accident in 1990
In this desolate setting, we are surprised to find toilets and changing tents have been set up, making our night the most comfortable yet and I look forward to a good sleep in this dark cocoon.
Heading out the next day, we climb the steep walls of the cave and wriggle through small gaps to reach the forest inside.
The still, cold air holds a purity like no other and the faint glimmers of light are cherished in the darkness, which we share with bats, spiders, fish and scorpions.
But the only glimpse I catch of these cave dwellers is the shadow of bats as they swoop down from the ceiling at dusk or are disturbed by the light of our head torches.
The moment our guide informs us there will be no more river crossings is a delightful one - and I stop to change into a pair of dry socks.
As we emerge from the cave, I watch the sun dip below the horizon once more and this time it's in full view.
After several days underground, it is a beautiful moment - but I would trade it for another night of darkness if it meant returning toSon Doong.
Laura visited Hang Son Doong with Oxalis Adventure. Visit oxalisadventure.com.
