From the air, these mysterious light-coloured rings of stunted tree growth are clearly visible, but on the ground, you could walk right through them without noticing them. They range in diameter from 30 metres to 2 kilometres, with the average ring measuring about 91 metres across. Over 2,000 of these forest rings have been documented, but scientists estimate the actual number is more than 8,000.
What causes these near-perfect circles in the forest? Since they were discovered on aerial photos about 50 years ago, the rings have baffled biologists, geologists and foresters. Some explanations put a UFO or extraterrestrial spin on the phenomenon. Astronomers suggest the rings might be the result of meteor strikes. Prospectors wonder whether the formations signal diamond-bearing kimberlites, a type of igneous rock...Although northern Ontario has the highest concentration of forest rings, you can also find them in the Yukon, Quebec, Russia and Australia..."
This CBC article discusses the currently-postulated most likely explanation for the rings.