Most substances are denser as a solid than as a liquid. Water, on the other hand, is denser as a liquid than as a solid. This is because when water freezes, its molecules form a crystalline structure that is less dense than the liquid water. The density of ice is about 917 kg/m^3, while the density of liquid water is about 1000 kg/m^3.

This unusual property of water has important implications for the environment and for life on Earth. For example, ice floats on water, which helps to insulate bodies of water and prevents them from freezing solid. This allows aquatic life to survive in even the coldest climates. Additionally, the expansion of water as it freezes can cause rocks to crack and break apart, which is a major force in the weathering of the Earth's surface.