Concrete is the second-most-consumed material on Earth, surpassed only by water, which is also a key ingredient in concrete. Its widespread use is largely due to its low cost and the local availability of its raw materials: sand, gravel, cement, and water. Concrete varies significantly from place to place, as its properties depend on the local materials used. However, this local variability can sometimes lead to unforeseen issues, like the problems faced by Fontana Dam in North Carolina. Completed in 1944, Fontana Dam is the tallest dam east of the Mississippi River, built primarily to provide hydropower. During construction, engineers took steps to mitigate thermal expansion issues by installing cooling pipes and expansion joints. Despite these precautions, cracks appeared in the dam less than 30 years later. Initial assessments blamed thermal expansion due to sunlight exposure, especially in the dam's curved sections. Further investigations revealed that the real culprit was alkali-silica reaction (ASR), a chemical reaction between the aggregates in the concrete and the alkaline pore water that leads to expansion and cracking. ASR is a significant issue in concrete structures worldwide, often resulting in costly repairs. The problem arises when certain types of silica in aggregates react with the alkaline environment created during cement hydration. This reaction generates a gel that absorbs moisture and swells, causing internal stress. Though challenging to avoid entirely, there are strategies to mitigate ASR by selecting non-reactive aggregates or reducing the alkali content in the cement. The Fontana Dam's case serves as a cautionary tale about the complexities of concrete construction and the importance of understanding local materials.