The Procellarum basin is thought by some to be the result of a gargantuan impact that occurred very early in lunar history. It is suggested that later impacts, such as Imbrium, would have partilly obliterated the outline of that ancient ''Gargantuan'' basin in most areas, leaving only its western margin with a circular shape. An alternative idea is that the Procellarum basin is the result of faulting associated with the Imbrium event. We have detected a subsurface horizon in Oceanus Procellarum using the Apollo lunar sounder experiment optical-format data. The horizon is deepest at the western edge of the basin, and shallows eastward. We did not observe peak ring structure in the areas where peak rings were predicted to exist according to the ''Gargantuan impact'' hypothesis. This observation implies that the alternative (graben) formation hypothesis for Oceanus Procellarum may be more likely. In this paper, we also review other geophysical and geologic evidence that supports the graben hypothesis. Finally, we report on some new radar evidence of a 500-km-diameter pre-Imbrian impact basin beneath Mare Insularum, in the Copernicus region. ¿ American Geophysical Union 1994