Pokemon Heart Gold and Soul Silver walk through?

Okay, so there's this small green house-like thing on Route 8 in the Kanto reigon. It looks like it could be the other entrance to the Underground Path right off of Celadon City. When you walk up and inspect the door and its sign it says, "It won't open! It's locked..." and "The notice is torn. It's impossible to read..." HOW DO I GET IN THIS DOOR?!?! Someone please help me out here, if there is a way to open it. :D