An iceberg is a piece of ice as big as a mountain. It is free to drift in the ocean without glacier or ice shelf. So looking at the shape floating on the water and not guessing the shape under the water. That's why the tip of the iceberg is said to describe serious problems only a small part of the surface. Icebergs are very strong, and the strength of metal in polar low temperature environment is reduced, which makes it easy to damage metal plates, so it is an extreme risk factor in polar ocean transportation.
The glaciers in the polar regions often form huge ice blocks at the mouth of the sea, which are broken by the impact of waves and float on the sea and become icebergs.