Both before and after players beat the main story of Paper Mario: The Thousand-Year Door, there are a handful of optional bosses that they can seek out for an extra challenge if they look hard enough. One of the first secret boss fights Mario is able to come across is the large Atomic Boo hidden somewhere within the Creepy Steeple. Players are able to seek him out as soon as they reach Twilight Town in Chapter 4 and have the opportunity to earn some great items and gear along the way.

Paper Mario: The Thousand-Year Door - Pianta Parlor Member's Card
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Paper Mario: The Thousand-Year Door - How to Get the Member's Card and Unlock Games in Pianta Parlor

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Where to Find Atomic Boo in Paper Mario: TTYD

Paper Mario: The Thousand-Year Door - Creepy Steeple

In Paper Mario: The Thousand-Year Door, players are able to locate Atomic Boo locked away deep within the Creepy Steeple. He's can be found as early as players' first visit to the area, even before encountering Chapter Four's boss.

Paper Mario: The Thousand-Year Door - Atomic Boo Location in Creepy Steeple

Upon entering the spooky church, players can make their way to the back of the main hall to find a statue of a star and some skid marks on the ground. Mario is able to push this statue out of the way to find the hidden catacombs underneath. Players are able to pass through here later in the story, but have access to it from the beginning.

Paper Mario: The Thousand-Year Door - Creepy Steeple Boo Box

Following the underground path to the left, you can begin the process of summoning Atomic Boo in a secluded room with a single purple box.

How Many Boos are in the Box in Paper Mario: TTYD?

Upon opening the small box, a torrent of Boos will startle Mario and fly out into the rest of the church. When they've all finally vacated the area, a single Boo will spring a pop quiz on Mario.

The Boo will ask how many total Boos escaped from the box as it was opened. Catching them off-guard, players most likely will not be actively counting as they pour out. The correct answer is that a whopping 200 Boos were in the box. For the correct answer, the Boo will reward Mario with an Ultra Mushroom. Depending on where players are in the story, they might not need use of such powerful healing and can always store this item for later.

Paper Mario: The Thousand-Year Door - Atomic Boo in Creepy Steeple

Back in the main hall, a single Boo cowers in the middle of the room. If spoken to, they'll ask Mario if he plans to be mean to them. If he says yes, the Boo will continue to cower.

Paper Mario: The Thousand-Year Door - Atomic Boo in Creepy Steeple

However, if Mario says no, the previous flood of Boos will immediately pour into the main hall. From this point on, Boos will appear in the main hall of the church to pester and battle Mario.

How to Fight Atomic Boo in Paper Mario: TTYD

As he walks through the Creepy Steeple, Boos will flock to Mario and surround him. If he stands still too long, he'll be unable to move, and the Boos will throw him out. In order to make Atomic Boo appear, Mario will need to use his Super Hammer's Spin Attack to scatter the Boos. Players should wind up their Spin Attack and let the Boos gather around them. They'll know they've waited long enough when the text bubbles begin appearing. Players will need to smack the Boos away twice in this way to summon the secret boss.

Paper Mario: The Thousand-Year Door - Atomic Boo in Creepy Steeple

Angry that Mario was mean to them, the Boos will form together to create Atomic Boo and attack. The boss has a solid 40 HP, an attack power of 4, and a slew of status effects it can inflict on Mario and the party.

Atomic Boo can stun or confuse Mario in addition to making itself invisible. Luckily, the Boos in the audience also have a random chance to hop on stage and make Mario invisible as well. With properly timing their Action Commands, players should also be able to avoid most of the status effects Atomic Boo tries to throw at them. Players should use all the stylish moves in their arsenal to overcome this boss battle.

Paper Mario: The Thousand-Year Door - Atomic Boo Reward in Creepy Steeple

Upon winning the battle, Mario will earn a 50-coin clear bonus and a decent amount of Star Points, especially if tackled early on in the game.​​​​

Paper Mario: The Thousand-Year Door - Atomic Boo Reward in Creepy Steeple

Afterward, the Boos will scatter and no longer swarm the area. They'll also drop the Lucky Start badge, one of the rarest badges in the game. Whenever Mario returns to the main hall of Creepy Steeple, Boos will now appear as normal enemies.