A fjord is a long, narrow inlet of the sea between tall cliffs or steep mountains. Fjords formed thousands of years ago during the Ice Age, when massive glaciers carved deep valleys through coastal mountains. As the glaciers melted and sea levels rose, ocean water flooded these valleys, creating the dramatic waterways we see today.

Norway is famous for its fjords, some stretching over 100 miles inland with cliffs rising thousands of feet on either side. The water in a fjord can be remarkably deep because glaciers cut so far down into the rock. Some Norwegian fjords are deeper than the surrounding ocean floor.

The word comes from Norwegian, where people have lived along these waterways for thousands of years. Similar formations exist in Alaska, Chile, New Zealand, and other places where glaciers once met the sea. Fjords often feature waterfalls cascading down their steep sides, and their calm waters can reflect the surrounding mountains like a mirror. Ships navigate carefully through fjords because the cliffs rise so steeply from the water, creating a narrow passage.

If you've seen pictures of ships sailing between towering green or snow-capped mountains with sheer rock faces, you were probably looking at a fjord.