Answer:
Icebergs are huge sheets of ice that caused the tragedy of the Titanic, etc.
Another Answer
An iceberg is a floating block of ice with an irregular shape. The ice was formed by frozen water moving downhill on a continent -- slowly, but rather like a river -- all the while compacting the minerals out of the ice and being built up increasing its thickness year after year by precipitation.
When the ice sheet 'flows' into the sea and part of the ice sheet floats against the tides, any weak spot in the ice sheet causes one of these blocks to 'calve' off the sheet -- separate -- and float into the sea.
Most of an iceberg floats underwater, hidden from view. Icebergs as large as some US states have been known to calve off ice sheets in the Antarctic.