From Bulbapedia, the community-driven Pokémon encyclopedia.
Frozen Island (Japanese: ひょうかいのしま Frozen Island) is a location in Pokémon Mystery Dungeon: Explorers of Sky featured in the Special Episode In the Future of Darkness. It is the bleak and dark future incarnation of Blizzard Island.
As the name implies, it is largely populated by Ice-type Pokémon. According to Dusknoir, Frozen Island is closed off in the future, with the actual dungeon being off limits.
Frozen Island no longer consists of the three "past" dungeons Blizzard Island, Crevice Cave, and Mt. Avalanche; it was instead replaced by three new dungeons, Dark Ice Mountain, Icicle Forest, and Vast Ice Mountain.
Locations on Frozen Island