The Electric counter — and a top offensive type.
Ground has the only immunity to Electric in the game, which makes it mandatory on any team that wants to switch into Volt Switch / Thunderbolt safely. It's also one of only three types that resists Poison. Offensively, Ground hits Electric, Fire, Poison, Rock and Steel for super-effective damage — the best Fire/Electric/Steel answer in one type.
Ground is a pivot and wallbreaker type. Sand teams use Excadrill and Tyranitar to abuse Sand Rush and Sand Force. Swords Dance Garchomp, Nasty Plot Hippowdon and Stealth Rock Landorus-T have been competitive staples for over a decade.
Earthquake is the iconic 100 BP physical move — every offensive Ground mon runs it. Earth Power is the special equivalent (90 BP + 10% Sp. Def drop). Precipice Blades is Groudon's signature 120 BP STAB.
Diglett was originally rumoured to be the small visible part of a much larger worm-like body buried underground — a fan theory that Game Freak officially canonised in Pokémon Sun & Moon's Alolan Diglett dex entry.