Everyone talks about underground path, and I don't even know where is it and how can I get in? Thats would be very help!
It's easy just find a open area(not inside a building)
and use the key item Explorer Kit.
Undergorund Path is in Kanto, between routes 5-6 and 7-8. In HG/SS, the one between 7-8 is closed (I don't know if it is closed in both IV and II gen)