Water actually has its maximum density at 4 deg.C, not zero. That is, it expands as it cools from 4 to 0. So very cold water (say at 0.1 degC) is least dense, and floats. That's why ice starts to form at the surface (that, and he fact that the water is losing heat to the colder air above). There is a further expansion on freezing as the water molecules adopt their positions in the ice crystal lattice.

Not only is 90% of an iceberg below the surface, it is more extensive as well as deeper. This, of course, is because if it was the other way up it'd fall over!

Not far from the North Pole, the Greenland ice sheet is so thick that if it alone were to melt it would raise mean sea level by 10m.   Or so I was told by a professor of climatology.

Steve.

PS Why don't we do something about stopping it happening?!!