The Brière marsh, covering about 14,224 hectares north of the Loire estuary, is a landscape of peat bogs, reed beds, and canals shaped during the 19th century. While many enjoy its serene beauty from a boat ride, this area holds a surprising and complex geological story. In the 19th century, local inhabitants, the Briérons, sought to transform this waterlogged region into arable land. They dug canals to drain excess water, making the soil firmer and more suitable for agriculture. At the time, this seemed like a success, as water levels dropped and the land appeared more stable and manageable. However, the Briérons were unaware that the soil was composed of peat, a type of organic material formed over thousands of years in waterlogged conditions. Peat is unique in that it can only be preserved if it remains submerged. When exposed to air through drainage, a chain reaction begins. Oxygen triggers bacterial oxidation, a process that has been causing the ground to sink for over 150 years. This degradation is not just a local issue but has global implications. Peatlands, though covering only 3% of the Earth's surface, store about 500 gigatonnes of carbon—equivalent to nearly seventy years of current human greenhouse gas emissions. This carbon remains trapped as long as the peat is waterlogged, but once exposed to air, bacteria break it down, releasing carbon dioxide into the atmosphere. The subsidence of the Brière marsh is not a relic of the past but an ongoing process. To monitor this phenomenon, the marsh now has a flux tower—a meteorological device placed above the vegetation that continuously measures the exchange of gases between the soil and the atmosphere. These measurements help determine whether the marsh is absorbing or releasing carbon on a daily basis. The Brière peatland is the largest in France, covering 14,224 hectares and containing 208 million cubic meters of peat. It ranks second nationally in carbon storage, making it a significant climate asset. However, it also poses a risk: if the peatland were to degrade completely, it could release 58 million tons of CO2 equivalent. This highlights the paradox of the Brière: a landscape that once stored carbon for millennia may now become a net emitter due to historical drainage and ongoing climate change. The degradation of peatlands is a global issue, with an estimated 50 to 75% of such areas destroyed or seriously damaged over the past two centuries. This makes the Brière marsh a rare and valuable site for study. The Regional Natural Park of Brière has launched a research program to determine whether its peatlands are currently acting as a carbon sink or a source. Initial core sampling has provided detailed data on the scale and condition of the site. The history of the Brière marsh illustrates a universal challenge: when humans drain peatlands, they transform a natural carbon reservoir into a potential source of greenhouse gases. As the waters rise in the marshes after the summer, the Brière paradox serves as a reminder of the long-term consequences of altering natural landscapes. Whether today's restoration efforts can reverse this trend and return the marshes to their role as carbon guardians remains to be seen.