Iceberg

An iceberg was a piece of ice that had dropped off of glaciers in the Arctic of Earth, or any planet with polar ice.

An iceberg struck the RMS Titanic (PROSE: The Left-Handed Hummingbird), an event which subsequently grew in Earth culture. The Fourth Doctor even commented that he didn't like the word "impregnable" because it sounded too much like "unsinkable." When Harry Sullivan asked what was wrong with "unsinkable," the Doctor replied "oh, nothing, said the iceberg to the Titantic." (TV: Robot)

The First Doctor trapped the Monk's TARDIS on an ice planet, where it disguised itself as part of an iceberg. (TV: The Daleks' Master Plan)