In Europe, they are classified as beneficial organisms, but many North American ecosystems are not adapted to these subterranean burrowers. This is because almost all earthworms became extinct there during the last ice age, which ended about 12,000 years ago. When the ice retreated, new ecosystems that are adapted to soils without earthworms emerged. But by now, several earthworm species live again in North America. They were introduced by European settlers and spread by anglers. An earthworm invasion is making its way through the forests at approximately five metres per year and is altering the physical and chemical properties of soils.