A berg is a large floating mass of ice that has broken off from a glacier or ice shelf and drifted into the ocean.

When glaciers reach the sea, huge chunks of ice calve off (break away) and become bergs floating in the water. These bergs can be enormous: the part you see above water might be as tall as a ten-story building, but about 90 percent of the berg floats hidden beneath the surface. That's where the expression “tip of the iceberg” comes from, meaning you're only seeing a small part of something much larger.

Bergs drift with ocean currents and winds, sometimes traveling thousands of miles from where they broke off. The most famous berg in history sank the Titanic in 1912. Sailors have to watch carefully for bergs, especially in foggy conditions, because crashing into one can tear open a ship's hull.

Scientists study bergs to learn about ancient climate patterns, since the ice contains air bubbles and particles from thousands of years ago. As bergs melt, they release fresh water into the ocean and sometimes carry rocks and soil far from shore.