It's a large floating rock, arctic animals like it and it can be placed in tanks or on normal water.
WARNING: Many people make this mistake- earning Excellence in Arctic Conservation (displaying beluga, walrus, narwhal, lion's mane jelly in suitable exhibits) will earn you a donated pair of emporer penguins.
To unlock the iceberg, you must earn the Excellence in Arctic Constuction award by displaying a beluga, walrus, narwhal, arctic wolf, polar bear, wooly mommoth, wooly rhino, and lion's mane jelly in suitable exhibits. You will earn $500 000 and the iceberg, wich will be available in the rocks tab.