An iceberg is an enormous chunk of ice floating in the ocean, broken off from a glacier or ice shelf. Most of an iceberg stays hidden underwater: only about one-tenth of its mass rises above the surface. This makes icebergs dangerous to ships, since what you see from the deck is just a small hint of the massive ice below. The Titanic famously sank in 1912 after striking an iceberg in the North Atlantic, a disaster that changed how ships navigate icy waters.

Icebergs form in polar regions when pieces of glaciers calve, or break away, and drift into the sea. They can be shockingly large: some icebergs are the size of small countries and take years to melt. Scientists study icebergs to learn about climate patterns and Earth's history, since the ice contains air bubbles and particles from thousands of years ago.

People often use the phrase “tip of the iceberg” to describe a situation where what's visible is just a small part of something much bigger. For example, if a few mistakes are found in a big project, they might be just the tip of the iceberg if many more problems are still hidden. The phrase captures how icebergs conceal most of their true size beneath the surface.