What happens when a layer of dense fluid is suspended over a less dense layer in a gravitational field, and then released? Rayleigh-Taylor instability.
The trick to observing this phenomenon is to arrange the two layers of liquid in place and then release them without interrupting the natural flow patterns too much.
I designed this apparatus as a postdoc at Cambridge University. The image also shows the salinity probe which was used to measure mixing efficiency as a function of time.
I then designed and demonstrated a radically improved version, based on a vertical bundle of drinking straws connected to a plenum, which could be used to hold the dense layer above the other layer until the moment of release.
(Here is a cool recent update using the equipment I designed in 1988).