These blockhouses from the Second World War provide a lesson in history and geography

At Pen Bron, in Loire-Atlantique, several bunkers of the Atlantic Wall are gradually sliding towards the sand under the effect of coastal erosion. Under the onslaught of swell, wind and rising water levels, these concrete colossi have become visible markers of the retreat of the coastline.

During February 2026, on the front page of the news were the floods, the rivers which swell and invade the villages. All eyes were on Sarthe, Mayenne and Maine-et-Loire. In Loire-Atlantique, the Loire was also becoming more comfortable and the Brière became an immense lake.

On the coast, nothing special, apparently. Because the work is done slowly, but surely. Tirelessly, the wind and swell, often coming from the west, attack the dune. A dune which starts from the tip of Pen Bron and joins the port of La Turballe, there in the distance. A superb dune, made of fine sand held by seaweed with long roots. Over time, these dune plants take hold and produce results, but faced with the rolling ocean and rising water levels, the fight is unequal.

In the silence of winter, a blockhouse “unscrewed” from the Pen Bron dune where it had been lying for decades. Faced with these events affecting our coastline, it was interesting to call on specialists to understand these phenomena affecting both history and geography.

Marc Robin, geographer, professor and researcher, specialist in coastal risks, from the University of Nantes, as well as Luc Brauer, co-director of the Grand Blockhaus museum in Batz-sur-Mer, talks to us about it.

During the Second World War, the occupier, fearing a landing, had thousands of bunkers built along the French and European coast to monitor and repel the enemy, all forming the Atlantic Wall.

In the town of La Turballe, dozens of blockhouses were installed in the retro-coastal zone for those housing heavy weapons and others on the coast for an immediate defensive response. The Pen Bron dune bordering a beach that could potentially accommodate a landing has been ”equipped” with several concrete sentinels.

“It’s really a small post, with a few blockhouses, blockhouses for machine guns, for mortars. You have to imagine that there were mines and barbed wire around. We’re talking about a small device in Pen Bron, but a device which extends around the port of Saint-Nazaire and which is a real fortress for the Germans, with around 1,300 blockhouses in the region”explains Luc Brauer, co-director of the Grand Blockhaus museum in Batz-sur-Mer.

These small bunkers have been monitoring the sea for decades. Built in record time with concrete and steel as ingredients. The colossi had sufficient weight to sit on them and had no foundation anchored in the sand.

With time, the wind, the rain, the waves, they were more and more updated, right down to their foundations. And then one day, there was no longer enough sand under these colossi which began to topple towards the beach. But as everything is happening slowly, one of the last ones still on the top of the dune at the start of the year is now at the foot of it.

“The base of the bunker rested on sand, but as the sea removes the sand, we have a scour which is created and therefore it tips over”, specifies Marc Robin, geographer, professor at the University of Nantes, specialist in coastal risks. The blockhouses therefore give the impression of being on the beach, but it is only an impression.

The bunkers therefore did not advance, at most they slid to the foot of the dune. For the rest, it is indeed this same dune which has retreated, which has lost its sand, carried away by the currents and the waves.

At Pen Bron, the geographer estimates that the dune has retreated by around 23 to 25 meters. This estimate was possible, among other things, thanks to these witnesses to History. Difficult, if not impossible, to move these masses of reinforced concrete, we are indeed in the presence of a geological evolution reinforced by global warming.

“These are markers of the erosive trend or the accretionary trend of certain coastal sectors. Here we are rather in an eroding sector, so they are found on the beach. In other places, we can find blockhouses which are still in the dunes, and even, where the dunes advance. If I take the example of the South Loire: Saint-Brévin for example.“, notes Marc Robin.

This small bunker, called Tobruk, has therefore left the top of the dune. His descent towards the beach began. In an undefined time, he will find himself in the middle of this beach, like his peers.

Only a few bunkers located right in the middle of the dune can hope to remain dominating the beach for a long time.

But in recent years, things have gotten worse. The ocean became more aggressive and launched its waves to attack the dune. With a swell and a westerly wind arriving directly at their target, particularly during repeated winter depressions, the erosion became regular, relentless and irremediable.

The latest blockhouse to have ”left” the dune will eventually find other fellows who ”left” long before him. With the point of Pen Bron on the back and the port of the Turballe in the line of sight, a new kind of demarcation line is obvious.

It is no longer the one that separated the free zone from the occupied zone during the Second World War, but rather a demarcation line of the 21st century. The one which marks the erosion of the coastline and the power of the elements.

Not everyone who walks on this Pen Bron beach during their vacation is a history buff. And these concrete cubes, there, on the beach, are for them only promontories to escape the waves, dominate the beach and admire the setting sun. It is no longer a question of the Atlantic Wall but of scattered and wobbly blocks having lost any impressive or threatening silhouette.

Such is life in blockhouses on our coast. Once threatening, even formidable, they have lost their aura but have gained another among children. Having become seaside adventure spots, they combine the functions of witnesses to History and witnesses to coastal erosion, and even global warming.

In a region like Aquitaine where the dune is even more attacked, certain blockhouse batteries can be visited with fins and diving masks. So let’s take the time to observe these bunkers lowered from the dune before the ocean swallows them.

Each time a blockhouse ”leaves the dune to reach the beach”, it is the end of one cycle and the beginning of another. First at the top of the beach, then in the middle, then, one day, underwater, the bunker modifies its environment.

Of course, the swell will collide with it, but it will also end up going around it and thus generating different currents. These will sometimes accelerate marine erosion which will attack the dune. We then decide to intervene either to protect the dune or to protect people. This was the case in Brétignolles-sur-Mer in Vendée around twenty years ago and in Olonne sur Mer in 2018.

The gaping holes left by the fallen bunkers are so many wounds into which the wind, the rain, the sea spray will slip. The municipalities are doing their best, as well as the Coastal Conservatory and the National Forestry Office, but everyone can do their part. For example, it is enough for walkers to take the indicated access points and not to multiply the descents on the beach, even if this is a delight for the children, and sometimes their parents.

Almost everywhere on the coast of Loire-Atlantique, Vendée, and elsewhere, the remains of the Atlantic Wall tell or recall the sad period of the Second World War, but they also tell of the modern era that we are experiencing through the prism of coastal erosion.

For those who often frequent the same beaches, walks can be an opportunity to follow this progression through photographic monitoring of the evolving position of the blockhouses from the dune towards the beach and, one day, towards the ocean.

Find us on our social networks and onfrance.tv