Mystery Mountain is a location first seen in the episode "Race Up Mystery Mountain".
The mountain is located in a place that can only be accessed through a portal in the city where the PJ Masks reside. To open the portal, one has to go to the wall the portal is hidden behind and use a ritual described in the Scroll of Mystery Mountain; an ancient scroll that was discovered in the vault of the Museum and promptly stolen by Night Ninja.
The ritual involves multiple hand movements, ending withdrawing the infinity symbol in the air.
Mystery Mountain is surrounded by a bamboo forest. It contains a pagoda on top, and various ornaments, all of which resemble ancient Japanese or Chinese architecture. The stairs leading up from the forest to Mystery Mountain are lit by lanterns.
The Pagoda contained the Ring of Ninjability, a ring that would give the bearer three times the power. Several booby traps were in place to protect the ring, including a coded lock on the door, dragon tongues and trapdoors.
In the series
On their first visit to Mystery Mountain, the PJ Masks competed with Night Ninja to get the ring first, but both failed. The second time Night Ninja forced the PJ Masks to get the ring for him by taking PJ Robot prisoner. This time they succeeded, but discovered the ring had no powers at all. What the ring and the traps were truly about was teaching any potential finders the value of teamwork.
Although he failed to obtain the ring, Night Ninja made Mystery Mountain into his new lair after these events. The mountain was next seen in "Gekko vs. Splatcano", in which Night Ninja transformed the mountain into a Splatcano, forcing the PJ Masks to once again travel to the mountain and stop him.
In "Wolfy Mountain", the Wolfy Kids discovered Mytery Mountain and tried to take it from Night Ninja, forcing the villain to team up with the PJ Masks.
In "Flight of the Ninja", which was also the last appearance of Mystery Mountain in Season 2, Night Ninja plans to launch a giant Sticky Splat onto the museum.
In "Armadylan Zen", which is also the first appearance of Mystery Mountain in Season 3, the PJ Masks take Armadylan on a zen trip in Mystery Mountain to help him to calm down, since Armadylan gets easily angry by Night Ninja and his Ninjalinos throwing sticky splats at him.
In "Werejalinos", the Wolfy Kids and their new allies, the Werejalinos, tried to take Mystery Mountain from Night Ninja, but the Wolfies ended up trapped by the dragon tongues, and the PJ Masks convinced the Werejalinos to give up being werewolves.
In "Teacher Goes Ninja", the PJ Masks went to stop Night Ninja and his Ninjalinos to rescue their [[Teacher]].
In "Meet An Yu", it is revealed that An Yu is the protector of Mystery Mountain. She tries to face Night Ninja by herself, but the PJ Masks helped her out anyway.