The underground path is a unique feature of Fulcre that allows you, the trainer, to travel different areas of Fulcre via the use of manmade tunnels containing pokemon. Sure these tunnels are long, but they are neccessary, as for one of these tunnels will lead you to pika-clone valley, aka the route outside of Lominto City, where you get your 5th charm. In these tunnels, you can find dark types, but types, maybe a ghost type or two, or even an eevee if your lucky.
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Key:
Dark Purple: Opening/Exit of an underground tunnel

Dark Pink: The path of tunnel

Dark Cyan dots: Special gates that keep you from going to places you're not supposed to go at this time

Purple Star: Places where legendary temples are

Dark blue dots: Gates leading under the sea to a temple
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Gates will be unlocked further into gameplay.