Kizingo Boulevard Vacation
From Halopedia, the Halo wiki
The Kizingo Boulevard Vacation is a glitch in Halo 3: ODST's Campaign level Kizingo Boulevard that allows you to get outside the map. Outside of the map is a humongous Da Vinci Square bigger than any other Da Vinci Code Object ever discovered.
To perform this glitch, you need to get to the part with the Gauss Cannon and the Type-26 Wraiths. You need to kill everything except the Banshee. You need to kill all the Marines, ODST's, and the enemies. Take out the Gauss Cannon very quickly so it doesn't destroy the Banshees. Once everything is dead, shoot a few bullets at a Banshee to make it follow you. Lure it into a corner and then it won't move. You need to shoot the Brute inside VERY slowly until it's shields pop. Once it's shields are down, shoot all of your ammo off so that you have almost a full clip, and have no extra ammo left. From here, hold down RB the whole time. While holding RB, VERY slowly shoot the Brute's feet with about 2-3 bullets at a time until it's dead. Once it's dead, you should immediately get inside the Banshee. Now, since the Banshee has a dead-man switch on it, DO NOT get out of it or else it will blow up. Now, to get out of the level, go to the area where the door is, and then boost into the wall on the number four while looking up. Try it a few times until you slip out of the level.
To see the Da Vinci Square, head out into the void, and boost for about 1–2 minutes. It should come into view, and then you can either stay in the Banshee and look at it, or get out, sacrificing your Banshee and walk around on it.