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 8 (?) 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.
This upgrade is useful against fully destroying everything below ZOMG health (including Fortified DDTs) easily in one ability explosion. In addition, it becomes handy at stunning all the ZOMGs on screen.
The massive damage that the Tsar Bomba nuke deals makes this upgrade very useful for beating Rounds 94, 96, 98, and 99.