This underground cave connects Route 2 and Route 11. It's full of Diglett and Dugtrio. They both have Arena Trap, an ability that doesn't allow you to escape from battle. If you want to avoid battles, pack a Flying-type Pokémon first in your party, since Arena Trap doesn't affect Pokémon immune to Ground-type moves. You can also use Repels, however, it only repels Pokémon that are at lower level than the first Pokémon in your party.
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Dugtrio at level 26. He has decent Attack and incredible Speed, but paper-thin defenses. He learns a powerful Ground-type move Earthquake at level 51.
If you want to catch one of these, it's recommended to catch a Diglett rather than Dugtrio. Although Dugtrio is much more powerful and comes at higher level, it doesn't have Magnitude in his arsenal due to level-up. Since Dugtrio learns Earthquake until at level 51, Magnitude is the only reliable Ground-type move before it (Mud-Slap and Sand Tomb are pitifully weak and Dig takes 2 turns to hit).