Water becomes ice at zero degrees Celsius, and it expands as it changes form. Snow is frozen rain and when it builds up it can be densely packed. Glaciers form from packed snow at the colder high altitudes and vast ice sheets form in the polar regions. Icebergs are created when chunks of an ice sheet break off or calve. The rate of ice melting is increasing, and the level of our oceans is slowly rising, threatening parts of the planet.