The subject is reviewed, notwithstanding the existence of a number of disagreeing and/or controversial results found in the literature. First, a brief critical reexamination of the methodology is presented. Second, it is attempted to put the results, or partial conclusions by different authors, using different methods and referring to different geographical regions, into a working scheme. This is done by investigating, as far as possible, the relationships between the electrical conductivity information and other types of geophysical and geological information for each geographical area investigated. It appears almost impossible to draw general conclusions that hold for the entire Earth. Conclusions are given for those areas with some very well-defined geomorphological characters. Unfortunately, the available investigations still appear to give a poor coverage of several types of geographic areas with specific tectonic characteristics, and certainly the scientific coverage by electromagnetic methods of investigation cannot be compared with those available today from seismological methods. Investigating the electrical conductivity structure of the lower crust certainly opens relevant heuristic possibilities, but there appears to be a great need both for a refinement in the basic methodology, and for a better coverage of the investigated areas.