Icebergs are large chunks of ice that break off from glaciers. This process is called calving. Icebergs float in the ocean, but are made of frozen freshwater, not saltwater. Most icebergs in the Northern Hemisphere break off from glaciers in Greenland. Sometimes they drift south with currents into the North Atlantic Ocean. Icebergs also calve from glaciers in Alaska. In the Southern Hemisphere, almost all icebergs calve from the continent of Antarctica.

Some icebergs are small. Bergy bits are floating sea ice that stretch no more than 5 meters (16.5 feet) above the ocean. Growlers are even smaller. Icebergs can also be huge. Some icebergs near Antarctica can be as big as Sicily, the largest island in the Mediterranean Sea.

As little as one-eighth of an iceberg is visible above the water. Most of the mass of an iceberg lies below the surface of the water. This is where the phrase "tip of the iceberg" came from, meaning only part of an idea or problem is known.

The ice below the water is dangerous to ships. The sharp, hidden ice can easily tear a hole in the bottom of a ship. A particularly dangerous part of the North Atlantic has come to be known as "Iceberg Alley" because of the high number of icebergs that find their way there. Iceberg Alley is located 250 miles east and southeast of Newfoundland, Canada.

In 1912, the Titanic struck an iceberg and sank in Iceberg Alley.  Soon after the Titanic sank, an International Ice Patrol was created to track icebergs and warn ships. That patrol continues today. Iceberg patrols now use global positioning system (GPS) technology to help locate icebergs and prevent more tragedies like the Titanic. 

Sourced from National Geographic Education