The wrinkly-eyed, leathery-skinned old man sucks back a large dollop of phlegm and expertly spits it far out the side of the 4×4 into the searing midday sun.
Fellow passengers tut and shake their heads as the silence is again filled with the sound of the engine furiously churning and gurgling beneath the murky water.
A small goat nibbling at a patch of dry grass on the sandy bank a few metres away momentarily stops and looks up at our windowless truck, which is currently perched in a precarious diagonal position, before casually turning his attention back to his lunch.
Holding my breath as the engine splutters and strains, and trying to ignore the puddle of water slowly gathering around my feet, I am beginning to see the motto emblazoned in the dune buggy’s front window, “Nas mãos de Deus” – “In the hands of God” in a new light.
Suddenly the truck lurches forwards and, with an almighty surge, pulls us free from what came close to being our boggy grave.
To a round of applause and cheers, we set back on our bumpy route through the marshes and desert of Brazil’s Lençóis Maranhenses.
The 155,000-hectare national park is filled with sweeping cream-coloured sand dunes as far as the eye can see, bordered by the Atlantic Ocean.
But it is the crystal-clear pools of rainwater that gather for a few months every year in the troughs of the dunes that we are hoping to witness.
Once we pull in at our destination of Barreirinhas, a sleepy, hammock-filled town on the banks of the Rio Preguiças (Lazy River), we are introduced to our guide, Mão, who is to take us on a 28km hike.
A friendly, cheeky sort of chappy, Mão has lived in the area for his whole life and seems to feel more at home in the desert, where he uses the sun and wind to help navigate his way across this stunning, desolate landscape.
Out on the sand, his legs seem to morph into superhuman gear and at several points during our trek, after pausing to take a picture, I will turn to say something only to find he is a rapidly disappearing dot on the horizon.
We learn that “Mão” is a nickname that translates as “Hand”, though we never did find out why.
As I pant and sweat while attempting to scale my first 40-metre dune, Mão shouts over his shoulder: “There are 10,000 water holes in the park, some up to three metres deep. You can’t come here without a guide though as it’s easy to get lost and your footprints disappear after about ten minutes with the wind.”
Not fancying that much, I hurry along to catch up.
At the top, Mão points ahead with a grin and my eyes soak up a vast, swimming-pool-blue stretch of water lying calmly below.
A little further on to the left sits another, emerald-green pool, while to the right a smaller, deep blue lagoon waits patiently.
Mão offers to carry our bags so we can dive in and swim to the other side of one of the lakes.
The water is as transparent and warm as the Caribbean Sea, but tastes deliciously fresh.
There is no-one else around, except the occasional Teteu bird circling overhead. From time to time it utters a shrill cry and swoops down rapidly to within a few feet of our heads, warning us to stay away from its nest.
Calf muscles aching after trudging through the powdery sand for a couple of hours, we arrive at a section of scrubby green vegetation dissected by patches of lilypad-topped water.
I shriek as I’m pulled down unexpectedly by a patch of quicksand, and, arms flailing wildly, for a split second find myself struggling to remember how Bear Grylls escaped from a similar situation on that episode I saw a couple of years ago.
But I soon stop sinking and eventually manage to pull both legs free. I bet Indiana Jones wouldn’t have worn Havaianas.
Mão says the only thing that’s ever been a victim of the “moving sand” is a Petrobras truck that came to investigate the oil presence here forty years ago and couldn’t be freed.
“There is oil here, but the company can’t touch it as the area’s protected now,” he explains. “They are trying to get permission though. Who knows what will happen in the future. They come here two or three times a year to inspect the wells.”
As it stands, during high season only a maximum of two hundred people are permitted to enter the park per day to preserve the area and its inhabitants – mainly black-skinned pigs, raposas (desert foxes), lizards, goats and turtles.
We stop for some food by a small waterfall cascading over soft, black sedimentary rock, and the tiny fish who have somehow managed to find their way into this arid land through the temporary water channels bite hungrily at our toes.
More marshy water leads us to our destination of Canto do Atins – population roughly half a dozen.
We arrive just as the sun is setting, washing a burnished orange colour over the land. I kick back in a hammock at our small pousada, whose roofs are lined with dried palm tree leaves.
The owner brings over a a Guaraná Jesus – a lurid pink, bubblegum-flavoured fizzy drink that is a favourite of all Maranhense kids – and a plate of juicy, fresh prawns, which I devour with gusto.
Tomorrow we will make the return trip to Barreirinhas by boat, past trees laden with monkeys and birds.
But my desert hike “Nas mãos de Mão” (“In the hands of Hand”) is an experience I won’t forget.