Two are near each other among the Nirgal Vallis riverbeds lying between the Valles Marineris canyon region, the huge Argyre Planitia
basin, and the dark Margaritifer Sinus albedo feature.
Yet the southern hemisphere is relatively featureless, revealing "no clear indication" even of the huge Hellas basin or the vast plain named Argyre Planitia
. Says Muhleman, "It is tempting (but dangerous) to say that this type of radar echo, whose physics is poorly understood, is primarily coming from relatively 'fresh' lava flows, except for the south pole." A great deal about Mars is yet to be learned, however, and Muhleman is not about to assert that the radar maps contain evidence for recent volcanic activity.