It has been found that low-frequency waving observed during tunneling microscopy with a stationary tip on liquid mercury is voltage dependent. This phenomenon can be explained by the Gibbs-Lippmann equation, which describes electrostatically induced changes in surface tension of mercury/electrolyte interfaces. The presence of adsorbing compounds overshadows the electrostatic effects, in complete agreement with the Gibbs-Lippmann model. These observations indicate that low bias voltages are required for maximum image resolution when performing tunneling microscopy at liquid surfaces.