Invasion in the Sea of Trees is the thirteenth mission of Metal Slug: 1st Mission.
The Hero traverses through a thick forest. The forest splits into two pathways: the upper path takes place on a bridge over the canopy, and the lower path is a denser forest. Both paths merge into one and quickly split again. The upper bridge (accessed by jumping over a tank) is the correct way out. Going the lower route takes the Hero back to the forest's entrance.
In the beginning of the forest, there is a tree with a hole on top of the trunk. Entering the tree skips to the final section, and a single bound to the right is enough to reach the upper bridge.
- Complete the Mission 10 (Night Flight)'s Parachute Drop
- Mission Complete: Proceed to Mission 14 (Weapons Port).