Shikkotsu Forest

is a legendary place that is equally famous as Mount Myōboku and Ryūchi Cave. It is the home of the slugs such as Katsuyu, which is summoned by the Fifth Hokage and at least one of her disciples Sakura Haruno.

Trivia

 * While both Mount Myōboku and Ryūchi Cave have shown multiple residents, the only known resident of the Shikkotsu Forest is Katsuyu. This could imply that the entire population of the forest is Katsuyu and her other parts.