The Ground Zero is the final upgrade for path 2 of the Monkey Ace, costing $11,900 on Easy, $14,000 on Medium, and $15120 on Hard. In Bloons Monkey City, it also costs $48,000 to research and requires the Super-Bomb Research Centre. It grants an ability that drops a nuclear bomb that deals 350 damage to all Bloons on screen, completely wiping out everything up to MOABs until fairly late rounds, and destroying the outer layer of the Dark Dirigible Titan (again, until fairly late rounds). It is comparable in power to the Super Monkey Storm, which has a very similar effect, but is a Special Agent.
Ground Zero used to do more than 350 damage