Humans use GPS devices and Google Maps to guide them home, but loggerhead turtles still prefer a magnetic compass. Unlike some beach bums, these turtles spend most of their lives in the ocean. Every 2 years, females cross the seven seas, docking on the beaches where they were born, to lay eggs. And although prior experiments have shown that loggerheads use magnetic fields to navigate in open water, mystery still surrounds how they find their native beach. Researchers report online today in Current Biology the first evidence that magnetic fields mark the spot for home—and even dictate its size. The team looked at 19 years of public data on turtle nesting locations in Florida and compared them with recordings of Earth’s magnetic fields. Much like shifting sand, magnetic fields slide slightly over time, and their strength also increases as one moves away from the equator, akin to latitude. This property gives each stretch of coast a unique geographic marker, known as an isoline. The team found that in years when these magnetic isolines moved apart, the turtle nests spread out over a larger area—by 1 or 2 kilometers. Conversely, when isolines converged, the nests squeezed into a smaller patch of beach, suggesting the turtles follow shifting magnetic tracks to their favorite nests. The findings also argue that a magnetic address is imprinted on loggerhead turtles at birth to point the way home.