The Mystery Box is a special item in Pokémon GO that temporarily causes the Mythical Pokémon Meltan to appear in the wild. You can obtain the Mystery Box by sending a Pokémon from Pokémon GO to Pokémon: Let’s Go, Pikachu!, Pokémon: Let’s Go, Eevee!, or Pokémon HOME.

Once you’ve activated the Mystery Box, you’ll have to wait three days and send another Pokémon to Pokémon: Let’s Go, Pikachu!, Pokémon: Let’s Go, Eevee!, or Pokémon HOME before you can activate it again, regardless of how you obtained it.