A device is described that excites individual modes of the standing wave spectrum for surface waves on water in a water tank of rectangular cross section. By synchronizing the downward acceleration of the tank with the standing wave on the water, Rayleigh–Taylor instabilities with reproducible characteristics are excited at the air–water interface. By controlling the width of the tank, the fluid motion is made to be largely two dimensional. The reproducibility of the phenomenon and the two-dimensional character of the flow make it possible, for the first time, to compare details of the wave profile with results predicted with the two-dimensional models of Emmons, Chang, and Watson.