In some places, like the Arctic and Antarctic, the temperature is so cold, most of the snow that falls doesn't melt. It just accumulates, layer upon layer, year after year - and gradually turns to ice.

Snow forms when water vapor condenses directly into ice around tiny bits of dust in the atmosphere, and the tiny particles that fall with the snow become trapped, along with gases, in the ice.

The deepest part of the ice in glaciers and ice sheets may have fallen as snow thousands and thousands of years ago, and the particles locked in it form a record of the Earth's atmosphere at that time.

By drilling or tunnelling into the ice and analyzing core samples, scientists can learn about past air quality, temperature changes, and types of vegetation.