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.
A foundation (also called a ground sill) is a structure that transfers loads to the earth. Concept : Foundation of a structure is always constructed below the ground level so as to increase the lateral stability of the structure.