A very, very large bomb. Someone put a stop to craziness!
Tsar Bomba is the final upgrade of Path 2 for the Monkey Ace in BTD6. It creates a much more damaging explosion that does 3000 damage that also stuns all Bloons excluding the B.A.D. (which can't be stunned at all) for 5 (?) seconds. Cooldown for the Tsar Bomba is 45 seconds upon activation. It costs $29,750 on Easy, $35,000 on Medium, $37,800 on Hard and $42,000 on Impoppable.