Icebergs are formed from ice breaking off of glaciers or ice shelves. Fluctuating temperatures and seismic events can cause the breaks in these glaciers and ice shelves.
Because of the sheer abundance of ice at the polar ice caps, icebergs are usually found in colder waters nearer the polar ice caps of the Arctic and the Antarctic, drifting along the natural currents in the ocean. When they reach warmer waters, they break up against shores or melt due to the higher temperature bombarding them from all sides. The largest icebergs are formed by breaking off of ice shelves; some icebergs are large enough to receive names and extended study to keep track of them.
Icebergs have been a threat to naval travel for as long as ships have traveled the regions they frequent. An iceberg's shape can often be deceptive, as a significant amount of the mass is often submerged and not visible. Icebergs can be strong enough to gouge huge holes in a ship's hull and sink vessels that fail to avoid them. The most famous example of this is the Titanic, a cruise ship that met its end in 1912 when it ran into an iceberg large enough to wound the hull.