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 a future version of Blizzard Island.
As the name implies, it is largely populated by Ice-type Pokémon. According to Dusknoir, Frozen Island merely closed in the future.
Frozen Island is no longer comprised of just one dungeon; it is replaced by 3 new dungeons, Dark Ice Mountain, Icicle Forest, and Vast Ice Mountain.
Locations on Frozen Island