Subterraneans

Race: Mysterious Being
Age: Unknown
Gender: Unknown
Location: Under the earth's crust
Abilities: Immense Strength
Affiliation: Mother Earth
Debut: Chapter 4/Episode 1

The Subterranean People are the self-proclaimed "True People of the Earth". They are ruled by the Subterranean King and live under the earth's crust, but grew too numerous in numbers and planned to take over the surface.

(Source: One-Punch-Man Wikia)