The Icy Pit is a small one-level dungeon in Act III of Diablo III. It only appears in Adventure Mode, and is guaranteed to appear in every game.
Its entrance is located in Eternal Woods. The zone's exact layout is random, but follows the same pattern (and has the same tileset) as the second level of Caverns of Frost or Icefall Caves.
A bounty may be called to clear this dungeon.