An iceberg was a piece of ice that had dropped off of glaciers.
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 Titanic." (TV: Robot) According to one account, the iceberg was actually the Monk's TARDIS, and the Titanic struck it when El Ape tried to ram it. (COMIC: Follow That TARDIS!)
Whilst on Platform One, the Ninth Doctor recalled boarding a ship which was said to be "unsinkable". He told Jabe, "I ended up clinging to an iceberg. It wasn't half cold." (TV: The End of the World)
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)
Aboard the SS Elysium, Michael Brack sculpted icebergs using a laser into the likenesses of passengers. (PROSE: Iceberg)
As a result of his previous incarnations being taken by Time Scoops, the Fifth Doctor felt that "great chunks" of his past were "detaching themselves like melting icebergs". (TV: The Five Doctors)