Collapse sinkholes commonly form in karstic limestones of the Floridan aquifer of southern Georgia, USA. The limestones are capped by impermeable strata that can obscure developing sub-surface voids and catastrophically collapse when too much of their underlying support has been removed. We investigated the overnight appearance of an island in a Georgian lake and its possible relationship to the underlying Floridan aquifer using ground-penetrating radar and global-positioning-satellite spot elevations. The island is adjacent to a submerged sinkhole with an arcuate depression and whose development included convergent downward slumping. Compression created by the convergence probably squeezed lake-bottom sediments upward to form the unusual island. Our methodology can be applied to other lakes in karst regions and may prove useful for diagnosing existing or future subsidence risks.