The mountain is located in another world, 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. Another way Mystery Mountain can be accessed is by air, as seen in "Flight of the Ninja". If one flies with just the right velocity, height, and perform the entrance ritual correctly, one will be able to travel to Mystery Mountain through a portal that's opened in the sky.
In various Season 3 episodes, the mountain can be seen hovering in the sky on a large cloud, something it never did in Season 2. One still needs the portal to reach the mountain though.
The ritual involves multiple hand movements, ending with drawing the infinity symbol in the air.
Mystery Mountain is surrounded by a bamboo forest. It contains a pagoda on top, a cave near the 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 on top of the mountain contains several relics of power. The first one to be revealed was the Ring of Ninjability; a ring that would give the bearer three times the power. Several booby traps are in place to protect the pagoda, including a coded lock on the door, dragon tongues and trapdoors. These booby traps remain a threat to anyone who visits the Pagoda, even after the ring was gone.
In "Meet An Yu", two more relics were revealed; A Red and Green Stone with the powers to turn anything to stone and turn invisible respectively. Night Ninja stole them, but An Yu took them back. Judging by the dialogue between the two, there were more stones inside the pagoda, but An Yu prevented the Ninjalinos from getting them. These stones and their powers have thus yet to be revealed.
The mountain contains several sources of red Sticky Splat, that can sometimes shoot up from the ground like geysers. The cave has a secret room filled with a special kind of sticky splat named Transforming Splat. In "The Splat Monster", Teeny Weeny Ninjalino used this splat to turn himself into a monster.
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 Mystery Mountain and tried to take it from Night Ninja, forcing the villain to team up with the PJ Masks.
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 "The Secret of the Pagoda", the PJ Masks work together with An Yu to stop Night Ninja and his Ninjalinos from stealing the Golden Wand from the pagoda.
In "Storm of the Ninja", the PJ Masks have to help An Yu to solve an ancient riddle to stop Night Ninja from forming a storm with a staff that would help him get to the Pagoda.
In "The Splat Monster", the Teeny Weeny Ninjalino discovers the secret room filled with a special kind of sticky splat named Transforming Splat in the cave of Mystery Mountain.