Crystal Walls

The Crystal Wall are a reflection of an individual's future self shown in the tomb of the first spinjitzu master.

Grave Danger
When Zane was walking by the crystal wall he notice himself looking older in the reflection. So did Kai and realise it's how they will appear in the future. When Jay looked at himself he's pirate and also discovers he ends up it Nya again. Cole tried to see himself but couldn't primarily the fact that he's a ghost.

Infamous
Jay was remarking his reflection to Zane. Zane questioned Jay on not mentioning it Cole. However Jay isn't willing to risk his friendship with Cole.

Trivia

 * Lloyd and Cole are the only two not shown what their future self looks like.
 * With the new designs of Season 8 it would seem that the Ninjas change in appearance cause a plot hole with the Crystal Wall reflection of their future selves.