First off, we need to get the difference between icebergs and ice-chunks.
Ice-chunks are frozen water.
Icebergs are frozen snow that builds up after a long time with out melting.
After a while gravity causes the frozen snow to slowly flow down the side of the iceberg at a grand speed of a few inches a day.
When the snow finally gets down into the water, it forms an ice-shelf. This ice-shelf is spread-out under the surface of the water. The biggest ice shelf was the size of Texas. This one was called the Ross ice-shelf.
Some icebergs are really big and a few where actually mistaken for islands. Well, the part of icebergs that we see is only about 10% of the actually iceberg and the other 90% is the part that we have to worry about. Sometimes the bottom of an iceberg can reach 800 meters or 2,620 feet. (This is down in the thermocline leval.)