Fungus Forest was a unfinished world that was planned to be released in Banjo-Kazooie. Due to insufficient time to complete the game, Rare did not feature the level in the game along with Mount Fire Eyes, Hammerhead Beach and two other mysterious levels. They instead reused the level in Donkey Kong 64 renaming it Fungi Forest. There is a photo of Fungus Forest in Banjo's House, on the wall.
It is speculated that Fungus Forest's level entrance is the Click Clock Wood jigsaw puzzle area. This is because the area's trees are covered with moss and fungus and the jigsaw is so far away from its entrance.
The believed entrance to Fungus Forest. The moss is clearly visible on the trees.