Laboratory experiments were carried out in a water tank at the Fluid Modeling Facility of the U.S. Environmental Protection Agency to study the rise and growth of buoyant thermals and their penetration through elevated density changes and into elevated stable layers. A dense volume of salt water (referred to as a thermal) was released at the top of the tank and observed as it fell (1) through water of constant density, (2) through a constant-density layer into a layer of greater density with a step-change and the height of the interface, or (3) through a constant-density layer into a layer with a linear increase of density with depth. Visual observations and video recordings were used to determine the rise and growth of the thermals as well as estimates of the fraction of the thermal that penetrated a step-change interface. Observations and concentration measurements were used to detemine the maximum penetration distance and equilibrium height of thermals that encounter an elevated linear gradient. A criterion was obtained for predicting when a thermal will penetrate a step-change in density as a function of its initial buoyancy, the magitude of the step-change and the height of the interface. The maximum penetration distance and equilibrium heights for the elevated gradient cases are presented graphically.