The low Reynolds number dynamics of a thin layer of fluid bounded below by a flat horizontal boundary and moving buoyantly through a fluid of another viscosity and density is observed by means of model experiments and is described theoretically. Three distinct stages of growth were observed. The first stage is described by a linearized Rayleigh‐Taylor instability, for which previous literature, present theory, and experiments exhibit close agreement. In this stage, disturbances of one specific wave number are found to grow most rapidly. In the second stage, distortion of the interface is large enough to invalidate the linearized analysis. It is found experimentally that the fluid moves upward as circular columns surrounded by relatively broad regions of descending material. A theory is advanced that attributes an accelerated growth to a structure of this kind through a resonant triad interaction. In the third stage, fully matured structures are formed. If the upwelling material has greater viscosity than the surrounding material, the structure is a long vertical column with gradually decreasing diameter; if the upwelling material has less viscosity than the surrounding material, the structure develops a rim syncline and a pronounced overhang and eventually ascends as a spherical pocket of fluid fed by a pipe. Dynamic explanations for these features are advanced, and some possible implications for geological and geophysical processes are discussed.