You cannot go unlock the Underground Path in HG/SS because you don't actually need it, since you can go through Saffron City.
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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!
you talk to prof. Elm who gives you a S.S. ticket to get on a boat from Olivine City to go to Kanto you can only do that after you have beaten the elite 4 and Lance the champion.
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.
After you defeating all the gym leaders in Johto and getting the Ho-oh or Lugia, go to your house in New Bark town. If you go right there is water. Use surf and keep going until you reach land. You will be in Kanto and i think a old man in the game tells you the same thing.