Podcast: Greenland's Mega-Canyon

In the age of GPS and Google Earth, it often feels as though every corner of our home planet has been mapped. At the very least, you think a satellite or two might notice a 450-mile-long canyon--twice the length of the Grand Canyon--poking out the northern end of Greenland.  But they didn't. Last week scientists at the University of Bristol in the UK announced the discovery of this so-called "mega-canyon," which remained hidden from human eyes because it's underneath a 2-mile-thick slab of ice....