https://www.polygon.com/pokemon-go/2018/11/16/18099225/meltan-pokemon-go-special-research

To commemorate the release of Pokémon: Let’s Go, Pikachu! and Pokémon: Let’s Go, Eevee!, the mythical Pokémon Meltan is available to catch now in Pokémon Go.

You will have to complete a series of special research to get a chance to catch the little gooey nut thing, similarly to how you have to for Celebi, Mew and Spiritomb. It looks like there are nine different steps to catching Meltan, which is the most we’ve seen so far.