Leads form between floes and these can be seen from a distance because they make clouds above them appear darker, a phenomenon known as water sky. In the nineteenth century sealing ships used this knowledge to assist in navigating through the ice. Occasionally large ice-free areas called polynyas form in the pack. These are thought to be caused by ocean currents pushing up warmer water. The largest iceberg ever sighted was a section which had broken off an Antarctic ice shelf and was more than 160 km long.
Many Antarctic icebergs are fragments which have broken off coastal ice shelves.
Because they are usually flat topped and of considerable area they are known as tabular bergs. Each year 35 km3 of ice is discharged from it. Icebergs from glaciers are usually much more irregular in shape than those from ice shelves. This would leave most of the continent with temperatures well below freezing, but the warming of the surrounding oceans could cause some of the floating ice shelves to disintegrate. Icebergs would then break off more frequently from the continental ice sheets.