Lake Flathead is a large underground lake that lies entirely within the caverns near the former location of the royal castle at Flatheadia.
To the west of the lake (yellow dock) lay the Royal Zoo; to the south (green) was the Great Underground Desert, to the north (red) was the Frobozz Philharmonic Orchestra Hall; and to the east (blue) was the Great Underground Mountain.
To the center of the bottom of the Lake there was a big red ruby which was perhaps was lost from the Oracle of Bargth. The Lake was also inhabited by squids.
The lake could be crossed via a magnificent royal Royal Yaucht operated by 4 colorful buttons, that never left that lake.