How is a phase diagram for water different?

A phase diagram is graphical representation that lets you work out exactly what phases are present at any given temperature an pressure. Every point in the diagram represents a possible combination of temperature and pressure for a closed system. Typically, a phase diagram has pressure on the y-axis and temperature on the x-axis.

The regions separated by the lines give us an idea of the conditions of T and P that are most likely to produce a gas, a liquid, or a solid. The lines that divide the diagram represent the combinations of temperature and pressure at which two states are in equilibrium.

Normally the solid/liquid phase line slopes positively to the right. However for other substances, like water, there is an anomalous behavior and the line slopes to the left. This indicates that the liquid phase is more dense than the solid phase. This phenomenon is caused by the crystal structure of the solid phase. Upon freezing, the of water decreases by about 9%. Because of that, you can melt ice simply by applying pressure and not by adding heat.