Jungle Dimension

"This place. I've been here before. I had a vision. I thought I understood what it meant, but maybe I was wrong. Maybe it was always about Emma becoming the Dark One."

- Mary Margaret Blanchard to Lancelot about the Jungle Dimension.

The 'Jungle Dimension is a jungle-like other dimension and one of many locations in the Once Upon a Time universe.

History
During his reign as king of Camelot, Arthur becomes consumed with finding Excalibur's missing half, the Dark One's Dagger, so much that he begins neglecting his own wife, Guinevere. When he thinks that he discovered the location of the dagger, Sir Lancelot prepares to leave to go there, but King Arthur insists he stay to protect Queen Guinevere, while he himself goes instead. That night, Guinevere finds a magic gauntlet in Merlin's tower that reveals the dagger's location as Arthur's greatest weakness. Intending to get the dagger, she tries to leave, before Lancelot agrees to accompany her. The gauntlet takes them to the Vault of the Dark One, and from one door in the vault, they head to the Jungle Dimension. There, they find the dagger on a table stand, but it's protected by a barrier. Knowing about Guinevere's quest to make the mystic blade Excalibur whole once more by uniting it with the dagger, Rumplestiltskin appears and he talks her out of trying to get the dagger and instead, offers her enchanted sand from the Isle of Avalon, that can give the illusion Excalibur is whole, In exchange, he wants the magic gauntlet she has. Rumplestiltskin cautions her to choose between duty and desire, which alludes to both Guinevere's marriage to Arthur and her feelings for Lancelot. In the end, despite Lancelot's protests, she accepts the deal.

Since learning from Maleficent that their unborn baby has the potential for both good or evil, Snow White and Prince Charming track down a unicorn to get a glimpse of their child's future. By touching its horn, Snow White sees their child, as a teenage girl, in the Jungle Dimension. Snow's daughter viciously rips out her mother's heart amidst protesting that she is the girl's mother.

Sometime after gaining this revelation from Lancelot, Mary Margaret helps him hide in the diner, keeping his return a secret. As Emma enters a catatonic state from the strain of being the Dark One, Mary Margaret, Hook, Regina and Henry are left unsure how to help her. When David arrives, he explains his conversation with Arthur, who can help Emma, but Mary Margaret reveals Lancelot's warning to her. After Henry leaves to take Emma, with Hook, to a place she can recuperate peacefully, Regina relates her suspicions that Emma was likely after the dagger and prepares to hide it, but David objects, stating that Arthur can help Emma if they give him the dagger. Mary Margaret stands by Lancelot's warning, however, David questions where the former knight has been. A frustrated Mary Margaret yells that she wouldn't know, as she doesn't have all the answers. Eventually, they devise a plan to test Arthur, by giving him a chance to steal a fake dagger. David leads Arthur to believe Mary Margaret has stolen the dagger, while Mary Margaret takes the dagger and goes with Lancelot to hide it in the Dark One's Vault. In one of the vault rooms which is the Jungle Dimension, she moves to put the dagger on a table stand, though it's protected by a barrier, but Lancelot insists on doing it instead, causing her to doubt his intentions. Arthur surprises them, getting Mary Margaret to forfeit the dagger to him, but after his attempt to summon Emma fails, she admits the dagger is fake, just before David ambushes Arthur.