The Big One

 is the final upgrade of the path 1 for the Mortar Tower. It costs $8500 (Easy), $10000 (Medium), $10800 (Hard), $12000 (Impoppable). It makes devastatingly large explosions, each popping through 5 layers of Bloon. This upgrade is unlocked at Rank 37.

Tips

 * This is useful if the player is facing Pink Bloons because it's strong enough to the point where pink bloons will be instantly destroyed the second the mortar hits them.
 * If the player buys this upgrade with Burny Stuff for both Mortar Towers on the Direct Assault mission just before round 23, they can pass the mission without spending a single dollar in Monkey Money.
 * This is extremely useful for rounds 21 and 36 and 47 with a Radar Scanner or a Meerkat Spy on round 47.
 * This upgrade isn't really effective on MOAB-Class Bloons, as it is like a slow-firing MOAB Mauler to them. You may want to damage them if possible.

Trivia

 *  replicates the Davy Crockett Weapon System.
 * In the Mobile version, it is not nerfed (meaning that it can still pop 5 layers). It appears that the description has an error in it. The description says "Devastating explosions each pop 3 layers of bloon.".
 * These are useful for making the game lag less because of how large the explosions are, with the huge range along with the fact that they take out more bloons instantly.
 * If the player adjusts the aim for a Mortar Tower with  on the center of Mt. Magma, or the center of 3 Times Around's circle, it will cover the entire circular center.
 * This is the only upgrade to increase the popping power of Mortars. Every other Mortar upgrade has a popping power of 40, while The Big One has a popping power of 100 (in addition to popping 5 layers, meaning the Big One can pop up to 3,100 bloons in a single shot (if it hits 100 Ceramics with 1 HP left).
 * The monkey next to  seems to wear the same clothes as a Glue Hose Glue Gunner, but the cloth is protecting the monkey from radioactive mortar shells and not from glue.
 * According to the upgrade picture, The Big One cannot safely be touched by the monkeys (even with gloves) without the help of tongs to insert the mortar. However the monkey is not using them in-game, so the upgrade picture could be purely for emphasis to show how dangerous The Big One is.
 * The explosion sizes are of the same of the missiles fired by a Temple of the Monkey God.
 *  is one of the few explosive towers that can pop black and zebra Bloons, the other ones being Bloon Impact, Hydra Rocket Pods, Spiked Mines, and a 2/3 Ninja Monkey. This is because they use Bloontonium.
 * This upgrade slightly increases attack speed.
 * The name may have come from the NASCAR term The Big One, meaning a huge crash.
 * It takes 40 Big One explosions to pop a M.O.A.B's outer layer
 * It makes a louder blast sound when its upgraded to this.