This very small cavern is located behind the waterfall. The walls are coated with ice that never seems to melt, and the whole place is freezing.
Rumours say that a rare pokemon was left here by her trainer over a thousand years ago, and waited there until she died. There isn't a lot here, though sometimes a trainer will wait here for a battle.
In the center of the cavern there is a strange rock, coated in ice like the everything else in the cave.
To get up here, you're going to have to have either a pokemon knowing Rock Climb or Waterfall.

[If an Eevee levels up here it will evolve into a Glaceon]