I was at least 20 steps behind my group – and another 30 behind our guide – when he suddenly stopped, checked his watch and tilted his face toward the sky, as if taking cues from the Sun.

"We must be lost," I thought. Smooth, pale slopes rose and fell in every direction, with glistening teal pools woven between them. It was a landscape with no obvious beginning or end. Then, as if reassured, our guide carried on, following a trail only he could see.

My three friends and I were a few hours into a three-day trek across Lençóis Maranhenses National Park, a humbling expanse of sand in north-eastern Brazil, and I had already lost all sense of direction. I walked in silence, listening to the wind, the rippling water, the sand crunching beneath my feet.

With each step, I sank a few centimetres, forcing my foot to work twice as hard. I kept falling behind, switching from flip-flops to water shoes and finally bare feet as the sand shifted from soft powder to a rock-hard surface baking in the heat. A friend back home had done a similar trek; "You'll feel bones you never knew existed in your feet," they'd warned. I was starting to believe them.