The process that occurs when two unequally dense liquids mix, with the heavier liquid pushing against the lighter one, e.g. due to the effect of gravity. A typical example would be a drop of thick syrup on the surface of water. During the dynamic mixing process, the two liquids try to reduce their combined potential energy. Rayleigh-Taylor instabilities occur in a wide range of situations and at all scales — from the mixing of intergalactic gas to the collapse of tiny bubbles during cavitation. In thermonuclear fusion research, Rayleigh-Taylor instabilities are an important phenomenon that mixes unequally dense regions of the plasma and can lead to its cooling.