Soccer ball

In the Halo 2 Metropolis (Level), there is a giant soccer ball hidden in the rooftops. To get it, retrieve the Scarab Gun and get back into the Banshee. Then head along the highway after the ampitheater until you find a building with a huge chunk taken out of it. At the bottom of the hole you will see the giant soccer ball If you manage to get it out before the Scarab comes by, it's really fun to see the Scarab kicking the ball around on its way to the canal. The Scarab Guns are good for pushing it around. There is speculation that it was put in there for Bungie to test the physics engine of the game, and they left it there for us to have fun with.

A red ball similar to the soccer ball is shown in the Limited Edition DVD. There is a section that talks about the special testing level the Bungie uses to test weapons, enemies, vehicles, enivronmental effects, sounds, etc. One of the scenes that is shown shows him driving a Warthog and moving the ball around.

Alternate method
There is actually another (not very well known, as me and my friend are the only people I have seen do it) way to get the ball. If you are not able to get the banshee to that part of the map, don't fret, all you need is to be in co-op and have a Wraith. Position one person at a certian position below a smaller building, but one still missing a chunk (Note: You will see it as soon as you walk back out into the street.), and the second person in the Wraith, with both people facing eachother. Have the Wraith boost. If the person not in the wraith jumps at the right time he will be boosted up into the broken part of the building. You are so close, but yet so far. Look around and you will see two metal things sticking out, one on each side. Jump onto the lower one, and after doing this, jump onto the second one. After doing so, jump up and on top of the building. Walk around untill you see a raised area with a fence type thing on it. It is missing a part an the wall, so jump up and through the hole and walk up to the building mentioned in the paragraph above and walk into it to get the ball. Just punch it and move it untill you knock it off the building and onto the ground. Get yourself down and into a second Wraith and you and your friend can play soccer with the ball and Wraiths...

Second alternate method
There is also one more way of getting the soccer ball down from the roof tops. If you know where the ball is located and just don't know how to get up there, follow these steps. When you come to the part when you can hit the Jackal in the back and steal his beam rifle, look for the gauss hog and go through that area, in the next part stay in the hog until your able to kill everything, but leave a ghost. Get in the ghost and ride up to the part where you recon with the marines (and eventually jump onto the scarab). Make sure you jump onto the Wraith and beat down the driver. Get into the Wraith and drive to the area with the ball. Fire Wraith shots into the area. This takes a very long time but it works. (This may require the Sputnik Skull; For I had the Sputnik skull when I did it, but I am not entirely sure. It would be much more wise to try the other two ways first.)