Secret Bonus Area #55 in The Mushroom ShoreEdit
The 55th Secret Bonus Area can be accessed by finding the entrance in The Mushroom Shore
The entrance is located at the top right of the zone[North of field 3]. It can be accessed as soon as you clear the blockade and can enter field 3. Find a small gap between the trees in the top right of the zone and follow the partially hidden path into the SBA.