Theory: Fungi Forest's Location

Arguments

Argument 1: Fungi Forest is the only level in Donkey Kong 64 where Steel Barrels appear in. While Steel Barrels also appeared outside of the Northern Kremisphere in prior games too, namely on Donkey Kong Island, they are much more common in the Northern Kremisphere. In Donkey Kong Country and Donkey Kong Land, both mostly taking place on Donkey Kong Island, only 29 Steel Barrels make an appearance. In Donkey Kong Country 3: Dixie Kong's Double Trouble! and Donkey Kong Land III, both set in the Northern Kremisphere, a total of 196 Steel Barrels appear (not counting the ones shot by Bazukas), giving the Northern Kremisphere by far the highest amount of Steel Barrels appearances.

Argument 2: The level is accessed through a portal in a cabin on a floating island that could impossibly house a large forest area. Cabins are also very prominent in the Northern Kremisphere.

Argument 3: Smaller red toadstools also appear in the Northern Kremisphere. The only other location featuring red toadstools was Crocodile Isle, which sunk into the ocean prior to Donkey Kong 64. No such toadstools could be found on Donkey Kong Island.

Argument 4: Just like the Northern Kremisphere, Fungi Forest features lakes, waterfalls, and smaller groups of conifers, highly resembling the landscape of the Northern Kremisphere. Even the bridges appear to be very similar to the ones found in the Northern Kremisphere.

Argument 5: One of the more prominent structures of the first worlds in both Donkey Kong Country 3: Dixie Kong's Double Trouble! and Donkey Kong Land III are barns and mills, with 7 out of 26 levels in the first two worlds taking place in them. This level type is exclusive to these two games, and mills and barns of this type and design only reappeared in the small area of Fungi Forest. And just like the mills and barns in Donkey Kong Country 3, bags of flour are found in them.