ScienceNow: Who needs a GPS when you have, well, whatever migratory birds and sea turtles have? For centuries, humans have marveled at the ability of these and other animals to navigate the globe, in some cases returning to the same breeding spot year after year from as far as 15,000 kilometers away. How they find their way remains a mystery, but new research suggests their prowess may depend on the ability to "see" Earth's magnetic field.