Recent cases of sinkholes in Normandy have once again highlighted the dangerous legacy of farmers who once mined the area for mineral fertiliser.
The practice – a little known part of France’s agricultural revolution from the late 18th Century to the 1880s – has left some areas at serious risk of sinkholes appearing without warning, often in gardens and beneath homes .
The mineral fertiliser is called marne, and is a form of chalky clay mixed with sand, which can be broken down easily for spreading on fields.
Marne was often found relatively close to the surface in thick layers across northern France, and especially in Seine-Maritime. Yields of wheat and other cereals improved significantly when it was used as a top-dressing.
Farmers, recognising they had a free source of fertiliser beneath their feet, began extracting it either through open-cast mining or by sinking shafts to the marne layer and excavating it, leaving deep cavities.
A sinkhole in Beuzeville in 2019DDTM 27
A tax was imposed on these marnières (chalk pits) in the 1850s, leading many farmers not to declare their activities.
When mining stopped, either because the marne layer was exhausted or because farmers switched to chemical fertilisers, they either marked the shaft, often with a tree, or filled it in, leaving a cavity.
It is these often-forgotten cavities that can collapse and lead to the formation of sinkholes.
Wet weather increases risk
The risk increases during wet winters. In one dramatic incident in 2002, an entire house suddenly slid into a sinkhole caused by a collapsing marnière in Rouen.
Recent cases have again highlighted the problem. Earlier this year, France 3 described how a homeowner in the commune of Grigneuseville, north of Rouen, unexpectedly discovered a hole three metres in diameter and eight metres deep beside his vegetable garden.
When he reported the problem to the mairie, it was found that the original cavity extended beneath the road outside his house. The road was subsequently closed and the homeowner was forced to vacate his property until the problem was resolved.
In France, homeowners are responsible for the subsoil under their property. This means that the cost of filling the hole, estimated at €15,000, fell on the owner and his household insurance policy.
“The insurers say they will honour the claim, but I will still be left having to pay €3,000, equal to 20% of the cost of filling the hole,” he said at the time.
The mayor of the village said the house was built 40 years ago on a field formerly used as pasture for livestock, and that no one at the time suspected there was a marnière underneath it.
A similar case occurred in 2025 in the commune of Rouville, also in Seine-Maritime.
While it is not a legal requirement to declare the risk of sinkholes prior to a sale, in this instance the notaire was informed independently by the mairie. Investigations identified two deep shafts on the property, with cavities remaining 30 metres below the surface.
The sale was cancelled, and the owner was required to spend €30,000 on geological surveys to determine the extent of the problem, before facing an estimated €150,000 bill to secure the cavities.
