Why do the iroquois call the muskrat earth-diver?

The Iroquois called the muskrat the Earth-Diver because a story has it that the muskrat dove into a deep pond and came up with a ball of mud. When it dried, it became the earth upon which Napi a mythical indigenous person molded the mountains and valleys and depressions for lakes and rivers and taught men and women how to hunt and live.