This is a female wasp in the family Scoliidae, probably genus Campsomeris. She is solitary, and will dig up a scarab beetle grub, sting it into paralysis, lay an egg on it, and leave. The larva that hatches from the egg will live as an external parasite on the beetle grub (which revives from paralysis but is eventually doomed by its parasite). So, this big wasp is good to have around in the garden! Won't sting you, either, unless you physically grab one.