An iceberg is a large piece of ice formed from freshwater that has broken off from a glacier or ice shelf and is floating in open water. It may subsequently become frozen into pack ice. Alternatively, it may come to rest on the seabed in shallower water, causing ice gouging in the land underneath or becoming an ice island. Because the density of pure ice is less than sea water an iceberg will float in sea water with about one-ninth of the volume of an iceberg above water. The shape of the underwater portion can be difficult to judge by looking at the portion above the surface. This has led to the expression “tip of the iceberg”, for a problem or difficulty that is only a small manifestation of a larger problem.