An iceberg is a large piece of freshwater 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, only one ninth of the volume of an iceberg is above water. The shape of the remainder under the water 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.

The word iceberg is a partial loan translation from Dutch ijsberg, literally meaning mountain of ice, cognate to Danish Isbjerg, Swedish Isberg, Low Saxon Iesbarg and German Eisberg.

Source : Wikipedia, image © Alfred-Wegener-Institut