I recently found this out. In Assault on the Control Room, when it begins, if you throw a grenade at the right moment when you are in the Pelican, you will stay in it and will travel to the bottom of the pit. Note I have only done this once by accident and it was on Legendary and co-op. After seeing Foehammer’s “expert” driving, you can get out and walk around and will find 3 grunts stuck together. They are in the old and new graphics, and will not move when yo hit them are grenade them, meaning they were especially placed there. After words you can flip the Pelican. Anyone else found this yet?
This is an old glitch.
You can also turn around and get back in IIRC.
You can also revisit the area via Banshee.
Also works if you keep mashing the board button.