This document analyzes an ancient masonry dam built in Italy in the 1920s on unfavorable geological conditions. Over nearly a century of monitoring, the dam exhibits significant seepage through the underlying morain deposit, causing cyclic horizontal movements that drag the dam's right shoulder. A geological model and careful back-analysis using soil mechanics principles reveal the complex interaction between the reservoir, subsoil, and dam structure. Proposed remedial works on the morain are not conclusively shown to improve dam behavior. Continuous monitoring and back-analysis remain essential for safe reservoir operation.