Ice Rings

http://commons.wikimedia.org/wiki/File:Circles_in_Thin_Ice,_Lake_Baikal,_Russia.jpg

What’s 2.5 miles wide, perfectly circular, and warm enough to melt ice?

I don’t know either, but there are at least two of them in Russia’s Lake Baikal.

They were spotted in April from the international space station.

09/26/2013 Resolved. (Thanks, Drew.)