damn you! ah hah, i win by elaborating!
it's a mine from the halo 2 multiplayer level Containment, which was recently released as part of the new multiplayer map packs. Containment was part of the free map pack, along with Warlock, a remake of Wizard from the original halo. if you have halo, take a sniper rifle and stand somewhere high up in either base. if you look over one of the cliff edges, you should see the rest of halo in the sky. look just above where half of the ring is dark and the other half light, and you should see an... interesting cloud formation.