The Thousand-Year Door
The Thousand Year Door is a massive door in Paper Mario: The Thousand Year Door that was said to have a treasure behind it. Generations of locals have created myths about what's behind it, and nobody knew that it turned out to be the entrance of Palace of Shadow. The only person that knows Rogueports true history is Grifty.
To open this door it requires all seven Crystal Stars to be placed before it. When Mario did this, the dark power caused the door and walls around it to crack. A vortex spewed out, and for the first time in a millenia, the Shadow Queen's stolen home was open.
Because of legends and probably a bad vibe from the area, very few people have even dared to touch the door. Intrestingly enough, the door seems thin because of it's height, but in comparison to Mario, it should be several feet thick.