From Bulbapedia, the community-driven Pokémon encyclopedia.
An Ultra Wormhole (Japanese: ウルトラホール Ultra Hole) is an unstable portal connecting the Pokémon world to Ultra Space discovered by Professor Mohn.
Not much is known about what causes Ultra Wormholes to form, but it is known that Cosmog can open it under heavy duress, at the cost of expending a lot of its energy, whereas its final evolutions Solgaleo and Lunala can open them more easily. Necrozma and its two other forms can also open Ultra Wormholes.
In Pokémon Sun and Moon, multiple Ultra Beasts enter Alola through a series Ultra Wormholes when Lusamine forces Nebby to open them all across the region. In Pokemon Ultra Sun and Ultra Moon, Necrozma opens multiple Ultra Wormholes across the Alola region after fusing with Nebby.
In Pokémon Sun and Moon, the player can travel through a spacetime rift at the top of the Altar of the SunneS or the Altar of the MooneM, which takes them to a parallel world where the time of day is opposite to their original world and with the Altar of the Sunne or Moone and the Lake of the Sunne or Moone replaced by their counterparts. They can only travel through this spacetime rift if they are the Champion and have Solgaleo in the party at night or Lunala during the day. In this parallel world, the spacetime rift is replaced by an Ultra Wormhole that the player can travel through at any time.
In Pokémon Ultra Sun and Ultra Moon, the player is also able to travel through Ultra Wormholes by riding on the back of Solgaleo or Lunala. This feature is known as the Ultra Warp Ride. While traveling through Ultra Space, the player can encounter Ultra Wormholes that lead them to the worlds of individual Ultra Beasts. They can also find warp holes that take them to locations that are light-years away that may contain Legendary Pokémon.
Some humans have passed through Ultra Wormholes, such as Anabel, and are known as Fallers (Japanese: Ｆａｌｌ). They have a strange energy about them that the Ultra Beasts mistake for another wormhole, causing them to pursue Fallers under the impression that they are the way back home. The International Police has quietly assigned Fallers as bait to lure out Ultra Beasts in their missions to stop them. Examples of this include an unnamed female Faller who accompanied Looker and Nanu on a mission to track down a Guzzlord, and the player in Looker and Anabel's missions to catch the Ultra Beasts.
In the anime
An Ultra Wormhole in the anime
Ultra Wormholes first appeared in A Dream Encounter!, where Ash saw two of them in a dream. He witnessed a Solgaleo and a Lunala each emerging from them one of them, and promised to take care of Nebby for the two Legendary Pokémon. These same wormholes reapparead in a flashback in Family Determination!.
An Ultra Wormhole was seen in flashback scenes in A Masked Warning! and Mission: Total Recall!, where it was opened up by Faba using a machine, and a Nihilego emerged from it. The Ultra Beast took notice of Lillie and proceeded to attack her. As it tried to take Lillie away, she was rescued by Silvally, who attacked Nihilego and sent it retreating back into the Ultra Wormhole, which then disappeared.
In Faba's Revenge!, Faba used Nebby's power to open an Ultra Wormhole, which a Nihilego entered through. The Ultra Beast attempted to take Lillie with it, but Lusamine protected her and Gladion, causing the Parasite Pokémon to take her with itself back through the wormhole, which closed in its wake.
In Revealing the Stuff of Legend!, Ash perfomed Searing Sunraze Smash with Nebby, who had just evolved into Solgaleo, allowing it to open an Ultra Wormhole for Ash, his classmates, and Gladion to travel though to Ultra Deep Sea, where they arrived in the next episode. After saving Lusamine from the alien world, the group returned back to their own world through another wormhole created by Nebby in 10,000,000 Reasons to Fight!. This wormhole was briefly seen again in a flashback in the next episode.
In A Mission of Ultra Urgency!, an Ultra Wormhole opened in the sky above Melemele Meadow, from which a Buzzwole fell through. After the Ultra Guardians had successfully captured Buzzwole, Professor Burnet temporarily opened a new wormhole above the meadow, allowing Buzzwole to return to its homeworld.
Prior to the events in SM069, an Ultra Wormhole opened in the sky above Bamboo Hill, from which a Celesteela fell through. It remained rooted down in Bamboo Hill for 200 years.
In SM078, a Blacephalon was revealed to have just entered Alola through an Ultra Wormhole. It performed in front of a group of people at a fireworks festival, during which the explosions it created caused another Ultra Wormhole to appear. From this wormhole, a Xurkitree appeared and started roaming around Melemele Island. Eventually, after the Ultra Guardians had successfully captured the two Ultra Beasts, Professor Burnet opened a new wormhole for each of them to return to their respective homes.
In the manga
An Ultra Wormhole in Pokémon Adventures
In the Pokémon Adventures manga
An Ultra Wormhole first appeared in PASM04, when Moon first spotted it in the sky near Mahalo Trail, just before saving Nebby from a flock of Spearow. Afterwards, when Gladion arrived late for the tournament at an Iki Town festival, he revealed that the reason for his absence was the same wormhole, which was spotted by and worried his Type: Null.
In PASM07, during Sun and Gladion's battle at the festival tournament, a large amount of energy shot from the Ultra Wormhole and entered Sun's Sparkling Stone. The energy allowed Sun and his Dollar to use a Z-Move to defeat Gladion's Type: Null and win the tournament.
In PASM09, when Moon and Mallow traveled to Lush Jungle, they witnessed Tapu Lele battling a Xurkitree emerging from an Ultra Wormhole in the sky above the forest. Moon attempted to capture Xurkitree with a Poké Ball-tipped arrow fired from her bow, but the Ultra Beast avoided capture and retreated back into the wormhole, which then closed shut.
When Mallow, Lana, Kiawe, Ilima, and Hau traveled to Po Town to talk with Team Skull, they found themselves surrounded by a Pheromosa, a Celesteela, and a Guzzlord, which started attacking everyone around them. When the Trial Captains' Pokémon started surviving longer than Guzma had expected, he revealed the Cosmog he had under his possession and had it start opening more wormholes in order to make more Ultra Beasts appear. One of the Ultra Beasts that came through these new wormholes was a Nihilego, which proceeded to grab Guzma and drag him back through the wormhole, which closed in their wake.
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