An iceberg (berg is the German word for mountain) is a large piece of ice that has broken off from a glacier or ice shelf and is floating in open water.
Typically, around 8/9 of the volume of an iceberg is under water, and that portion's shape can be difficult to surmise from looking at what is visible above the surface. This has led to the expression "tip of the iceberg", generally applied to a problem or difficulty, meaning that the problem is only a small manifestation of a larger trouble.
The mass can be very durable and can easily damage sheet metal. As a result of these factors, icebergs are considered extremely dangerous hazards to shipping. The most famous sinking from an iceberg collision was the destruction of the RMS Titanic on April 14, 1912.