Geochemical observations demonstrate that Earth's mantle is heterogeneous, but the sizes, forms, and characteristics of its reservoirs are not constrained. Using laboratory experiments, we have systematically studied a simple nonhomogeneous system, where two layers of miscible fluids with different densities, depths, and viscosities are subjected to a destabilizing temperature contrast. When the buoyancy number B (i.e., the ratio of the stabilizing chemical density anomaly to the destabilizing thermal density anomaly) is lower than 0.3--0.5, the whole layer regime develops, with a deformed interface and convective patterns over the whole tank depth. The system systematically evolves toward one-fluid Rayleigh-B¿nard convection because of stirring. However, the two isolated fluids can persist for very long time compared to the characteristic timescale of thermal convection and give rise to numerous transient behaviors. Of particular interest for planetary mantles are the pulsatory dynamics, where the interface between the two layers deforms in large domes moving up and down quasiperiodically: According to the scaling laws derived from the experiments, such a mechanism could indeed provide a simple and single physical explanation for the superswells observed at present on Earth and more generally for the long-term episodicity in planetary interiors observed in geological records.