A glacier is a massive, slow-moving river of ice that forms over many years in places where snow accumulates faster than it melts. Picture a mountain valley where snow piles up winter after winter, getting packed down under its own weight until it transforms into thick, dense ice. Eventually, this ice becomes so heavy that gravity pulls it slowly downhill, grinding and carving the landscape as it flows.

Glaciers move so slowly you can't see them creeping forward, but over decades and centuries, they reshape entire valleys, carve out lakes, and polish rocks smooth. Some glaciers are small enough to fit in a single mountain valley, while others, called ice sheets, cover entire regions. Antarctica and Greenland have ice sheets so enormous they hold most of Earth's fresh water locked in frozen form.

When glaciers reach warmer areas or the ocean, chunks break off in a process called calving, creating icebergs. Scientists study glaciers carefully because they act like frozen time capsules, trapping air bubbles and particles from long ago. These ancient ice layers reveal what Earth's climate was like thousands of years in the past, helping us understand how our planet changes over time.