Forest ring
Forest rings are large, circular patterns of low tree density in the boreal forests of northern Canada. These rings can range from 50 metres (160 ft) to nearly 2 kilometres (1.2 mi) in diameter, with rims about 20 metres (66 ft) in thickness. The origin of forest rings is not known, despite several mechanisms for their creation having been proposed. Such hypotheses include radially growing fungus, buried kimberlite pipes, trapped gas pockets, and meteorite impact craters.