Mystery Box

The Mystery Box (Japanese: ふしぎなはこ Mystery Box) is a special item in Pokémon GO. It is used to spawn wild Meltan.

History
According to Professor Willow, the Mystery Box was made by people who worshiped Melmetal to keep it safe. When the Pokémon splits itself into Meltan, they are stored in the box until it opened again in order for Melmetal to appear. The box will opened when exposed to open air in an natural environment.