The Rebel Underground Drill is the huge drill that appears in Metal Slug 3D. It is the 5th Boss.
This huge drill was sent to the underground to break the SV-001 base. In the beginning of the cutscene, Allen tells to Marco that will destroy the SV-001 base, and Allen escapes, with the Rebels Ready to kill Marco. You need to destroy all of its parts to make the drill stop. If you hold longer and the drill completely enters the hole, you fail the mission.