Geographic location of the bug: San Diego coastal 15″ below ground
Time: 08:00 PM EDT
Your letter to the bugman: I’ve never encountered a grub so large before and would like to know what kind of beetle this will become.
How you want your letter signed: Matt Lee
This appears to be one of the Root Borers in the subfamily Prioninae. Was there a tree or shrub nearby, or perhaps the trunk of something that had died? While we are reluctant to provide a definitive species identification, it might be the larva of a California Root Borer like this image posted to BugGuide. According to BugGuide: “Larvae: Up to 80mm long” and “Larva feed primarily on living deciduous trees (oaks, madrone, cottonwood) and are also recorded from roots of vines, grasses, and decomposing hardwoods and conifers. Will also attack fruit trees growing on light, well-drained soils (e.g. apple, cherry, peach).” If that is a correct identification, here is an image of an adult male California Root Borer, though your larva might belong to a different, though similarly large Prionid with long antennae.