Very rare and very popular Legendary Pokémon are returning to Pokémon Go as Special Research encounters.

Pokémon Go is a highly successful mobile game based on the top-rated Pokémon franchise. Pokemon Go allows players to capture Pokémon in the real world. The game has been a hit since it came out in 2016, having accumulated around 3 billion dollars’ worth of revenue since launch.

Special Research are unique tasks that players can complete in the game, and completing these tasks will reward gamers with prizes. Special Research can also reward players with special encounters with rare Legendary Pokémon.

Niantic officially announced that they are offering five Legendary Pokémon as Special Research Encounters. The five Legendaries are Articuno, Zapdos, Moltres, Kyogre, and Groudon. These five Legendaries will be available in Special Research starting on Nov. 1 at 1 p.m. pacific daylight time. They will stop being available on Jan. 1, 2020, at 1 p.m. PDT.

Trainers, we have some news!✅ Articuno✅ Zapdos✅ Moltres✅ Kyogre✅ GroudonAll will be available as Research Breakthrough encounters for November AND December! Learn more: https://t.co/vRgWgzu3Hx pic.twitter.com/AKgSkjgJ7p

— Pokémon GO (@PokemonGoApp) October 31, 2019

Articuno, Zapdos, and Moltres are the Legendary bird trio from the original Pokémon Red and Blue games. Articuno is a bird with the powers of ice. Zapdos is the master of electricity, and Moltres is the Legendary flame bird. Their types are self-explanatory.

Kyogre and Groudon are the masters of the sea and land, respectively, first introduced in Pokémon Ruby and Sapphire. Kyogre is a giant whale-like Pokémon and is a pure water-type. Groudon is a pure-ground type unless it turns into its Primal form; Primal Groudon is a ground/fire-type.