The Doomberg is how the inhabitants of Emperor-Land nicknamed the huge iceberg that blocked the only entrance and exit of their territory. The Doomberg is the main antagonist of Happy Feet Two.
History
At the beggining of the film, it is shown how some drops of water fall in a crack in the ice. This crack get wider and wider, until a huge piece of ice was separated from the continent. Later, the piece of ice went on a collision course with Emperor-Land and crashed into it, blocking its only entrance and exit.
Mumble, with the help of the Adelies penguins (Summoned by Boadicea) and the elephants seals (Headed by Bryan), destroyed parts of the iceberg by dancing, creating a kind of tunnel and the trapped penguins escaped throught it.