A crack in a wall, a door that no longer closes properly, or tiles that seem to lift near a window—these small issues are affecting millions of French homes. While many homeowners assume these problems are due to aging or harsh weather, the real cause is the movement of the ground beneath their homes. This movement is driven by seasonal changes in soil moisture, particularly in areas with clay-rich soil. As summers grow warmer and drier, this phenomenon is becoming more widespread, impacting over ten million individual homes across France. The process, known as clay shrinkage and swelling, occurs when clay soil alternates between absorbing water and drying out. When the ground is wet, clay expands, pushing up the foundations slightly. In dry conditions, the soil contracts, creating voids beneath the structures and causing them to settle unevenly. This subtle but persistent movement leads to cracks in walls, often appearing as diagonal lines near windows and doors. These cracks can even widen or appear to close again depending on the season, making the damage seem intermittent. The Southwest region and the Paris Basin are among the most affected areas, especially older homes with shallow foundations. These homes were often built without modern soil analysis, making them more vulnerable to the effects of clay movement. Additionally, large trees near homes can worsen the problem, as their roots dry out the soil around the foundations, increasing the risk of shrinkage. While some might view these cracks as minor aesthetic issues, they can signal serious structural problems that may require costly repairs. The financial impact of this phenomenon is significant, with annual damage claims reaching several hundred million euros. Insurers face complex claims due to the gradual nature of the damage, which can delay the repair process for homeowners. As climate change brings more frequent and intense dry summers, the risk is expanding even to areas previously considered less vulnerable. In response, new construction now more commonly includes geotechnical studies and soil-adapted foundations. For existing homes, solutions like irrigation systems around foundations and careful tree placement can help mitigate the risk. However, these measures are still not widely adopted. As the climate continues to change, understanding and addressing this hidden threat will be crucial for the future of housing in France.