Peat in certain wetlands in the Pas-de-Calais has been trapping carbon for nearly a thousand years, particularly in the Audomarois Marsh. This natural process occurs because the waterlogged, oxygen-deprived soils prevent the decomposition of organic matter, allowing carbon to be stored instead of being released as CO2. However, this mechanism is now reversing, with carbon being released into the atmosphere as CO2. Drainage linked to agriculture and forestry has already rendered about 40 percent of peatlands inactive, causing the release of stored carbon. Better water management, through the restoration of natural water levels, could significantly reduce global CO2 emissions from drained peatlands. The Audomarois Marsh, located around the town of Saint-Omer in the Pas-de-Calais, is a Ramsar site recognized for its major ecological importance and covers about 22,300 hectares. It is shaped by a unique geography at the crossroads of the coastal plain, the inland Flanders, and the Artois hills. Nationally, France is home to about 100,000 hectares of peatlands. A two-meter-thick peatland can contain up to 1,400 tons of carbon per hectare. Since medieval times, the waterlogged soil of the Audomarois Marsh has functioned as a real carbon vault, trapping organic matter before it fully decomposes. The secret of peat is the absence of oxygen. As long as the soil remains waterlogged, organic matter does not decompose normally but fossilizes slowly, trapping carbon for hundreds or even thousands of years. However, when the environment is drained, the peat mineralizes and decomposes, and the carbon it contained transforms into CO2, which escapes into the atmosphere. This mechanism is documented by Ramsar France experts and is currently a concern, as the release linked to the mineralization of drained peat is estimated at between 2 and 3 gigatons of CO2 per year, a volume comparable to the emissions of entire industrialized countries. The history of the Audomarois Marsh illustrates the ambivalence of human action on these environments. For generations, inhabitants have drained wetlands for sanitation purposes or to free up arable land. Peat was also extracted as fuel for decades before the ecological consequences were understood. When the hydrological trend reverses, usually due to drainage, it marks the end of the peatland as it was known. The environment gradually dries out, the decomposition of fossilized organic matter resumes, and the carbon stored for centuries escapes in considerable quantities of CO2. The Audomarois Marsh is not an isolated case, as other French peatlands, such as the Vernier Marsh in the Eure, the Saint-Gond Marsh in the Marne, or the Lavours Marsh in the Ain, experience similar dynamics depending on the human pressures they face. The case of the Pas-de-Calais is part of a broader dynamic, with peatlands potentially shifting from carbon sinks to emission sources by the end of this century, a reversal already observed in Switzerland. International studies have focused on estimating carbon emissions linked to the drainage of agricultural peatlands over the last millennium. The conclusions suggest that better water management of these soils, particularly through the restoration of natural water levels, could significantly reduce global CO2 emissions. This would not only involve preserving intact peatlands but also rehumidifying those that have been drained.