A laboratory basin of saltwater with a freshwater layer at the top is exposed to cooling from above and inflow of freshwater from a layer that lies alongside. It exhibits oscillations of temperature, salinity, and layer depth when certain parameters are set. A simple analytical model illustrates the basic mechanism. Upon cooling, the layer gradually becomes denser. Ultimately, a density inversion at the interface between the layer and deeper fluid causes a gravitational instability. The layer is mixed with the deeper water and it vanishes. The emplacement of a new thin layer from freshwater inflow at the top repeats the cycle. Layered numerical models duplicate this cycle. Examples incorporate transport of heat out of the top and into the bottom of the layer. In addition, there is salinity transport up through the bottom from turbulent mixing. They reproduce the qualitative structure of the experimental data, but the many simplifications in the model do not permit precise quantitative agreement.