An iceberg is a large piece of ice that has broken off from a snow-formed glacier or ice shelf and is floating in open water. Since the density of pure water ice is ca. 920 kg/m3, and that of sea water ca. 1025 kg/m3, typically, around 90% 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 visible trouble is only a small manifestation of a larger problem.