Diglett's Cave
From Pokémon HeartGold & SoulSilver Wiki
Diglett's Cave
Until you have Rock Climb this tunnel is a one way move. Just go forward or back... there is no other way to go.
Snorlax Blocking the Way
Blocking the way of the Diglett's cave is a big Snorlax which you can wake up via the Pokeflute Channel on your PokeGear Radio. It will battle you and be at Lv. 50 giving you the chance to capture this awesome Pokemon.
In order to remove Snorlax from your way, you will have to complete a series of events first. If you go to the Radio Tower in Lavender Town, you will be told that they've no electricity and when you go to the Power Plant you will be informed that they are currently missing an essential machine part.
After this if you go to the Cerulean City Gym, a Rocket Grunt will run into you and then leave. He will then be located on the Nugget Bridge and after a battle, will inform you that he left the machine part in the liferings in the Gym. After you retrieve it, and bring it to the Power Plant, they will give you the Kanto Radio Card in the Radio Tower in Lavender Town.
Now you are finally able to battle Snorlax. Simple go to the radio function of your PokeGear, and manually tune it to the top of the circle. After this, press A at Snorlax and the battle will begin.
