What's behind door one?

Hint 1 of 6:

What does it look like?

Hint 2 of 6:

Maybe it's another maze,

Hint 3 of 6:

or some more secret hallways.

Hint 4 of 6:

You might have been here before,

Hint 5 of 6:

from the garden water basin.

Hint 6 of 6:

In fact, you're in the mines right now, but you don't have to be here yet. 
First explore the areas behind the other doors in the planetarium room. 
There are some important things that should be done first.