An Atomic Boo is a large enemy in the video game Paper Mario: The Thousand-Year Door and Super Paper Mario. In the game, he would appear if Mario smashed the Boos surrounding him twice. Thankfully, the enemy is an optional boss that Mario doesn't in fact have to fight. An Atomic Boo will form when exactly two hundred regular Boos attach to each other. Once they do, the fight will start. If Mario is able to defeat the boss, then all of the Boos in the Creepy Steeple will leave for good, and the plumber will obtain a special badge. In Super Paper Mario, an Atomic Boo appears in Merlee's Basement with 10 HP. Thankfully, Atomic Boo's are easy to avoid in Merlee's Basement.