Secret Exit - Pass through the Midway Gate, come out the first pipe, make your way through the rising and falling ceiling maze part, and go through that pipe. Now, you have to be Caped Mario at the part where you see your first Mega Mole. Kill him with your cape, build up enough speed and fly up and to the left onto the top of the ceiling. Continue to the left until you drop down to the Key.
so basically kill that first mole, go back a bit to get enough flying speed, then fly up and over to the left.
