Surface-to-air missile launcher

In the Halo 3 multiplayer level, High Ground, there appears to be a large three barreled mortar. Bungie staff have called the weapon the Big Ass Mortar Gun.

Nothing is really known about the gun apart from it appears to play a significant part in the Level High Ground maybe acting like the Automatic Turrets in Snowbound which shoot players venturing outside map borders. That is very possible, because this picture shows a beach area marked with the text "Mortar Area".