Mother Nature Network: Whats weirder than a slick, oily substance suddenly appearing at a northwest Indiana beach on Lake Michigan? Its equally sudden disappearance. Swimmers at Porter Beach at Indiana Dunes National Lakeshore came out of the water Monday afternoon to find themselves covered in an oily substance and there was a silver sheen on the lake, according to the Coast Guard and the Indiana Department of Environmental Management (IDEM). But by the next day, it was gone. "They checked the beach, and they can't find...