Steelix

Steelix is a Ground/Steel type Pokemon discovered in Generation II. It is the evolved form of Onix. This can be done by trading it away while it's holding a Metal Coat.

Steelix lives even further underground than Onix. This Pokémon is known to dig toward the earth's core. There are records of this Pokémon reaching a depth of over six-tenths of a mile underground.