THE DOOR RIDDLE

You and your friends came to a large mountain to hike. It is covered with rocks, stones, and looks like it has been through a landslide. However, there is a door right in front of you. The door is polished, and looks brand new. You can still see the fresh wax on it.
''Hey dude," says your pal Billy.
"Yeah?"
"I dare you to go inside."
..
"But it's weird, I mean, who polishes a door and sticks it in the middle of a cave?"
"You doing it or not, man?"