Castillo de San Marcos, near Saint Augustine, Florida, is the oldest existing masonry fortress constructed of coquina stone. Its construction was started in 1672 and many modifications were made until its final deactivation as a fort in 1900. Gradually, the exposed surfaces of the exterior stone have weathered and some of the most severely deteriorated stone has been replaced. The fort is exposed to seasonally heavy thunderstorms and sea water spray. Coquina is a porous stone which can be easily penetrated by water and, as a consequence, the walls appear damp and in some areas efflorescence is observed. Historic records indicate these problems have existed for many years, but the matter is of serious concern for the National Park Service (NPS), which plans to have Castillo structurally stabilized by 1992. The director of NPS instructed the Southeast Cultural Resources Preservation Center to develop a low-cost and intensive monitoring program for the monument to determine pathological conditions of building materials, causes of deterioration, structural conditions, and causes of possible structural movements.