Discovery of a Secret Passage is the ninth mission of Metal Slug: 1st Mission.
The Hero hops on a SV-000 to traverse a cave system. The first half of the mission has a small shortcut that is accessed by destroying a wall and then jumping on two platforms that fall together.
Destruction of the SV-000 leads to Prison Camp. The camps at this point become more elaborate. The first part is crawling through tunnels and navigating inside a building, the second part is jumping on platforms over a large gap, and the final part is running through a roof with spikes popping out. If the Hero misses the jump to the final section, a hallway with Rebels is available below by hugging the wall.
- Complete Mission 6 (Penetration of the Front Base).
- Mission Complete: Proceed to Mission 10 (Night Flight).
- Prison Camp Complete: Retreat to Mission 4 (Underground Battle Pass).