Minerva's Door

Minerva's Door is a secret door hidden beneath the windmill at Goldy Pond. The door can only be opened with a Promised Pen. The door is located beyond a secret passage in the windmill and down a spiral staircase and then down a ladder leading deeper into the third layer of Goldy Pond.

Beyond the door lies the control room for Goldy Pond and the Golden Lake with the floating cabin on it. In the cabin, there is an old elevator and an old telephone. It is revealed that the elevator once led to the Human World, but has since been deactivated. The elevator can only be accessed by using the Promised Pen. The book, Adventures of Ugo hints at how to find the pen cap for the Promised Pen. During a pre-recorded phone call from Minerva inside the floating cabin which contains the elevator at Goldy Pond, he told the escapees that he left "the rest of the details" in "Marvine's bed". In the book, Marvine sleeps inside of a desk drawer, alluding that the pen cap was hidden in a nearby desk drawer.

Appearance
The door has William Minerva's owl icon on it surrounded by a circle of Morse Code which decodes to "Pen" indicating that the Promised Pen is the key to opening the door.