It's super easy finding the Diglett in this zone, since it's extremely short and there's barely any places to hide. Maybe we should have started here, as a warm up? Anyway, just relax while we send these wayward sheep home.
1/3: From the entrance, it's along the left after turning the first corner. It's preeeetty hard to miss.
2/3: Halfway through the tunnel, turn towards the small passageway along the left. Before entering the passageway, check behind the rocks on the left.
3/3: Make your way towards the exit. After turning the last corner, it's waiting for you on the right, just before the beginning of Potbottom Desert. Score!
That makes just over 100 Diglett rescued! Now you can receive a starter Pokémon all the way from the Alola region. The one you get depends on the Type of your Galarian starter: Rowlet if Grass, Litten if Fire or Popplio if Water.