In the small town of Trans-en-Provence in the Var region of southern France, Belgian engineer Achille Knapen built a unique structure in 1931. Designed to collect atmospheric humidity through condensation, the structure resembles a beehive and is made of a central mass of concrete mixed with porphyry gravel and cement mortar. This central core is covered with a thin layer of mortar where oblique slates are embedded. A thick masonry wall, 2.50 meters wide, surrounds this core, enclosing an empty space at the center known as the "aerial" well. This well was meant to capture water from condensation. The construction of the structure took one and a half years before it was put into operation. Knapen’s design relied on temperature differences to create condensation: during the day, hot air would enter the dome and accumulate, then cool at night, causing water vapor in the air to condense. The structure was intended for use in desert regions, where large temperature swings would make the process more effective. However, the Var region did not provide the necessary temperature differences for the system to function well. According to the city of Trans-en-Provence, the failure was due to insufficient nighttime temperature variations and an oversized condenser. Knapen had expected much greater temperature contrasts, but during the summer months, the differences were only a few degrees. For a condenser to work, its surface must drop below the dew point of the air—the temperature at which vapor turns into liquid droplets. The oversized design likely made it difficult to achieve this threshold, as the amount of material needed to cool was too large for the minimal gain in condensation. Today, the structure is a historical monument, inscribed by a decree on December 9, 1983, and protected as a heritage site by the Ministry of Culture. It also carries the label "Heritage of the 20th Century," awarded by a circular on March 1, 2001. Despite its status, the structure may have never functioned as intended, as noted in the Mérimée database, maintained by the Ministry of Culture. The building is still visitable, and its dome, covered with oblique slates, is a popular spot for photography, especially in low light. The visible dome is only the outer shell of the system, and the many holes drilled in the wall allow air to circulate. Interestingly, Knapen had retired to Trans-en-Provence in 1930 and built a villa at 180 meters altitude on a nearby promontory, just next to the experimental site.