Fig. 7a-d. Equipotential surfaces (meridional sections) for the marginally stable () configurations of test perfect fluid orbiting the Schwarzschild-anti-de Sitter black holes, and the related potential , given for . The behavior of the equipotential surfaces has the same character for all . There are four possibilities described in the text. We express the radial coordinate in units of M, and use the logarithmic scale. The central black hole is shaded. Notice the special shape of the equipotential surfaces with a cusp, resembling a falling wave. The dashed lines give asymptotics of , and determine the interval of radii where the equipotential surfaces cannot reach the equatorial plane.