We constructed detailed geologic maps of channels within three widely separated regions using Magellan synthetic aperture radar (SAR) imagery and altimetry radar data in order to understand the operative processes involved in Venusian channel formation. Channels in each region cut topographic highs including ridges and shield edifices with no deflection of the channel course by surface topography. We argue that these relationships are difficult to reconcile with the widely held view that Venusian channels represent surface processes, whether constructional or erosional. We conclude that the channels evolved through subsurface fluid flow, which involved local stoping and transport of overlying material; that is, these channels were carved from below, rather than from above. We postulate that at least some Venusian channels form because of subsurface fluid flow along a shallow crustal interface that forms the boundary between overlying low backscatter surface materials and underlying basal materials. Fluid movement along the interface may facilitate piecemeal stoping and erosion of the local surface materials from below, eventually resulting in the formation of channel traces exposed at the surface. The low backscatter material which hosts the channels in this study also hosts abundant coalescing shields and associated deposits, which may represent shield terrain.