An iceberg is a huge chunk of ice that breaks off of a glacier and floats around the ocean. Most of an iceberg is under water and can't be seen; that's how a sneaky iceberg managed to sink the Titanic.
Icebergs exist in the coldest waters on the planet — the largest icebergs ever observed have been in Antarctica. An iceberg is blamed in the famous sinking of the Titanic in 1912, and the disaster inspired a new system for tracking and measuring icebergs. Another kind of iceberg is the lettuce, which is crisp and nearly as pale as an actual iceberg. The colloquial "tip of the iceberg" means the smallest hint of a much larger problem.