An ice age is a long period in Earth's history when thick sheets of ice covered large parts of the planet. During these frozen eras, glaciers (enormous rivers of ice) spread across continents, covering areas that are now forests, cities, and farmland. During the last ice age, ice sheets over a mile thick covered Canada, the northern United States, and much of Europe.

During ice ages, so much water freezes into glaciers that ocean levels drop significantly. Animals adapted to the cold, like woolly mammoths and saber-toothed cats, roamed landscapes that look completely different today. Early humans lived through the last ice age, hunting these animals and taking shelter in caves.

Scientists have discovered that Earth has experienced several ice ages throughout its history, each lasting millions of years. Within ice ages, there are colder periods (when ice advances) and warmer periods called interglacials (when ice retreats). We're living in an interglacial period right now.

The term ice age is sometimes used casually to describe any extremely cold period, but geologists use it specifically to mean these major glacial periods that transformed Earth's climate and geography.