Heffalump Hollow is the main home for heffalumps in the Hundred Acre Wood. Heffalump Hollow is separated from the rest of the Woods by a large fence. For the most part it is a densely forested area, so it can be quite dark inside even in the daytime.
Up until Pooh's Heffalump Movie, Pooh and his friends avoided Heffalump Hollow because they were scared of the heffalumps inside. In turn, the heffalumps that lived there avoided the rest of the Wood, believing that scary creatures lived there.
Heffalump Hollow Returned in Lumpy's Adventures.