Catastrophic sector collapse occurs when a volcano becomes structurally unable to support its own load. One process particularly capable of weakening the edifice is hydrothermal activity. It can produce high pore pressures and alter strong rock to clays. Alteration can extend progressively over long periods (>100 yr), allowing deformation to develop slowly before collapse. An important finding is that structures produced by such deformation are recognizable and could permit collapse prediction. We present the case of Casita, Nicaragua, where hydrothermal activity has been weakening the edifice core, causing flank spreading, altering original constructional shape, and steepening flank slopes. One side is slumping outward, producing a crescentic scar with a basal bulge. We identify this feature as the site of a potential sector collapse, with conditions ripe for failure.