Midway Forest is a forested area located between Mt. Colubo and the Gray Terminal. It is known for being a dangerous place to any human.
Portgas D. Ace and Sabo used this place as a sort of hideout in their youth, hiding the loot they obtained from stealing within a hidden storage in a large tree. A young Luffy was able to locate them and their treasure stash after following Ace through Mt. Colubo.
After defeating Bluejam in the burning Gray Terminal, Ace and Curly Dadan took refuge next to a river in the forest in order to avoid the spreading fire.