This underground cave connects Route 2 and Route 11. It's full of Diglett and Dugtrio. They both have an ability that doesn't allow you to escape from battle, so you might want to use Repel when you cross this cave. Two basic repels will get you through the cave one way with some steps to spare. Use four total for a round trip.
The second Ground-type Pokémon that you can catch in Pokémon FireRed. He is very useful against Electric-type Pokémon, but can be quite useless against flying ones. He is incredibly fast but has a very little HP. He can learn many Ground type moves like Magnitude, Earthquake and Fissure.
Diglett
Dugtrio