Underground Battle Pass is the fourth mission of Metal Slug: 1st Mission.
The Hero traverses through an underground sewer avoiding the Rebels, piranhas, bats, and latching rats. If the player completed Mission 3 by the second hidden exit, the Hero enters this mission from a later point. The various lights surrounding the sewer's walls block the player's projectiles and dims the stage as the light dissipates.
- Access the Hidden Routes of Mission 3 (Battle in the City).
- Incomplete Mission 9 (Discovery of a Secret Passage).
- Mission Complete: Proceed to Mission 10 (Night Flight).