If you're talking about the one in Kanto, then I'll tell you. You have to get snorlax out of the way using the pokeflute channel on the radio. You then have to battle him. You can faint him, catch him whatever. You will then be able to use the underground path!
Hope this helps!
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You cannot go unlock the Underground Path in HG/SS because you don't actually need it, since you can go through Saffron City.
Try to find the Underground path in Goldenrod city , Then travel throught that path , the you should find a Team Rocket guy , He will give you the Team Rocket outfit . *Hope it helped , Kruh .
Ok had to think for a sec. There's this guy in the Underground Paths...one of the two. After you get the electricity back on go through the underground paths and talk to this bald guy. Make sure you have a RageCandyBar from Mahogany. He gives you explosion for it. Pretty sure its the Celadon Underground Path.
The bf in the department store and in the underground hall thingy in goldenrod city
The card key in Pokemon Heartgold and Soulsilver is located in the Goldenrod Underground.