What is behind the old man's door in Mossdeep City? (Pokemon Ruby/Sapphire/Emerald)?

Question

It only says it's locked.

Answer 1

The locked door can only be opened after you activate the Mystery Event. Then, you need to use the E-Reader, a link cable, & a second GBA to swipe the card that came with the game. After that happens, the trainer on your card will be in the room.

Comments

Here is a picture of it:
http://google.com/m/search?site=images&source=mog&gl=us&q=pokemon%20sapphire%20mossdeep%20door#i=3

The 4th one on the top right.