Mining. See this link for example http://www.npr.org/blogs/krulwich/2013/01/16/169511949/a-mysterious-patch-of-light-shows-up-in-the-north-dakota-dark http://w

Mining. See this link for example http://www.npr.org/blogs/krulwich/2013/01/16/169511949/a-mysterious-patch-of-light-shows-up-in-the-north-dakota-dark http://www.npr.org/blogs/krulwich/2013/01/16/169511949/a-mys...