Ghost Forging

Ghost Forging is a glitch in Halo 3 that allows a player in Forge to fuse an immovable object into the ground or other objects in real time.

Walkthrough
First, you have to have at least two of the objects that you want to ghost forge. Next, go into forge mode. Press up on the D-Pad and X at the same time. If done correctly, you will have the item list while in player mode. This may take a few tries.

Once in the item list and in player mode, go to the object you want to ghost forge and press A. Repeat for the same object so that you will have two of the same objects on the field.

Go to one of the objects and set its respawn time to never. Then grab the other object and wait for the first object to disappear. Once the object that was set to never disappears, then press X and set the runtime minimum to the runtime maximum.

Move away, and you will notice that you can move the object through the ground and other objects, it can also be placed in air and will stay there.

The final thing to do is to start a new round and delete the object you set to never. It will be where you spawned it at the beginning in the air.