There is no underground because you are in a different region. Only Sinnoh region has the underground, You have to find shards and heart scales in rocks. To do that you have to use rock smash.
The card key in Pokemon Heartgold and Soulsilver is located in the Goldenrod Underground.
No. The underground is only in Pearl, Diamond and Platinum.
You can't mine underground in either Pokemon Heartgold or Soulsilver, but you can mine underground in Pokemon Diamond, Pearl, Platinum.
There is no where to get the Explorer Kit in Pokémon HeartGold because there is no underground stuff in the game.
you can not go undergrond
By going underground and digging.
the underground tunnel in goldenrod
The lady behind the counter in the underground tunnel in goldenrod city.
The Underground is unavailable in HeartGold and SoulSilver versions
Go to goldenrod city and go to the underground tunnel then talk to a guy there and he will give your pokemon a haircut
Sorry. You can't. It was never intended to be passable
powering up the power plant and near it is zapdos