An iceberg or ice mountain is a large piece of freshwater ice that has broken off a glacier or an ice shelf and is floating freely in open water. It may subsequently become frozen into pack ice (one form of sea ice). As it drifts into shallower waters, it may come into contact with the seabed, a phenomenon called seabed gouging by ice. About 90 percent of an iceberg is below the surface of the water. Icebergs are possible on Earth because of an unusual property of water: It is less dense in its solid state than its liquid state. Oceans on some other planets consisting of other types of liquids, such as methane, cannot have icebergs, precisely because the frozen substance would sink.