This can only be unlocked on the zombie team in Gardens and Graveyards mode in Cactus Canyon. As a team you need to defeat the plants at all the gardens until you reach the part where you have a golf cart bomb to push into the hole.
You need to work as a team to not only push the cart along the track towards the hole but, to defend the bomb against the plants team.
If you succeed in reaching the hole, a cutscene will play out destroying the plants base.