induration. 1. the process of hardening or being hardened.
1 They're weak against the Ice and Water types in reference of erosion, which cause this elements to the ground. 2  They're immune against the Electric type because in real life, the ground is a natural sink of electrons.  The Bug type resists them because many bugs like worms live inside the ground and many of them eat it.
In rocky areas, where it is difficult to drive a ground rod 8 feet down, a grounding plate may be used. This thick piece of metal is buried underneath a footing or foundation. Or a ground wire may connect to a metal reinforcing rod embedded in a house's concrete foundation.