Majin Buu's Cocoon is a big brown ball with veiny-things on it. When it opens, it splits in half shooting smoke out.
The cocoon was hidden on Earth by Bibidi. When Babidi found it, he took it to the bottom of his spaceship. After Spopovich and Yamu drained the energy of Gohan at the World Martial Arts Tournament, Babidi almost had enough energy to revive Majin Buu. Using the warriors of his spaceship, and energy from the fight between Goku and Vegeta he gained sufficient energy from the Z Fighters, and the deadly Majin was awakened.