THE Dune du Pilat looks, when approached from its base, less like a dune than an imposing wall of sand. From below, as we approached it from our campsite, it loomed over us, seemingly almost vertically, the only clue to the fact it was climbable being the ant-like chains of people that dotted its surface. Some 107 metres high and three kilometres long, this dune, Europe's largest and France’s second most-visited natural monument, looks like a piece of the desert transplanted onto the French Atlantic coast. It dwarfed the campsite in which we had hired a chalet below. It also represented the perfect challenge for two boys who told me, as they stood peering up from its foot, that they wanted “an achievement”.

Some way along there was a set of stairs, a sloping metal ladder, pinned onto the sand, but my kids were having none of that. They were staring straight up, which was the way they wanted to go. They wanted to take on the dune itself, the sand monster, without help. This meant some walking and some crawling, in the late afternoon heat, straight uphill towards the hot blue sky.

All along this stretch of Atlantic coastline, near Arcachon, there are signs telling people not to jump and run on the dunes, fences cutting off patches that are badly eroded. But here, no-one seems worried about conservation of the Dune du Pilat. After all, it is growing, slowly but surely getting bigger, and steadily creeping inland, by between one and five metres per year, as sand is blown from the shore by westerly Atlantic winds. In the last 100 years it has doubled in size. Often it is caricatured as a kind of slow-moving, devouring sand beast: houses and pine trees have been engulfed by this sand. Pilat, meaning a heap or pile in the local dialect, has long been its name. Maps from the 18th century place areas called Pilat to the south and offshore of the dune's present location. The area where the dune currently sits was referred to as Les Sabloneys until the 1930s, when it was renamed the Dune of Pilat.

Give it a few years and the tent pitches closest tents to the dune will probably have been covered. Wait long enough and the entire campsite and road beyond it may be eaten up. The map here is always changing. And this sense of the dune as almost alive and in motion adds to the drama of climbing it.

The ascent on the landward side is tough. Feet sink deep into the sand. Pockets fill up. Shoes overflow. Soon we were taking them ours and feeling warm sand between our toes. The grains here are the finest in all of France: from 0.25 to 0.3 mm. Despite having done a 10K run only weeks before I found I was having to tackle it in runs of 30 or 40 steps before stopping to rest. My eldest son darted ahead and reached the top first, where he disappeared for a moment over the edge before peering back down at us. Over the other side is a long, gentle descent to the sea; a strong breeze whipping sand into the air at the cusp; a sky dotted with the brightly coloured arcs of paragliders, who regularly use the dune as a leap-off.

My sons celebrated reaching the top with an imaginary medals ceremony, as we stared down towards the inviting azure of the Atlantic. Out there in the sea is the Arguin sandbank, often described as "bird island", where, in spring, sandwich terns gather to make their nests. For boats, this stretch of water, with its shifting sands, is one of the most dangerous on this coast.

From the summit of the dune to the sea it is as if you march through desert, tripping down over the soft undulating sandscape.

Needless to say we climbed this dune more than once. A favourite pursuit was to haul ourselves the up steep side and roll or tumble down again. Though there were some facilities – a swimming pool, a kids club, a play area – at the campsite (Camping La Forêt, one of several along the stretch of the dune), there was endless entertainment in the dune itself, the real fun being in the descent of the steep side. Some children surfed down it on boards. But my sons loved nothing better than a swimming race, freestyle or crawl, with arms digging into the sand to propel us down the slope.

The sand gets everywhere. Into pockets, in underwear, into the seams of your clothing. By the time one reaches the bottom of the slope, a few kilogrammes have been gained in sand, though possibly a couple have been lost through the sheer effort of having got to the top in the first place. This a sand monster that gets into you, stays with you long after you have left the coast – always reminding you of itself in grains found here and there.

www.ladunedupilat.com or www.dune-pyla.com/en/