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If you splashed down in the Atlantic, you'd flounder on which way to swim. But a hatchling loggerhead turtle would know just where to paddle—by reading the Earth's magnetic field. Scientists knew turtles can pinpoint¹ latitude² this way, because the field varies a lot from north to south. But not east to west. So how do turtles know which side of the Atlantic they're on?

To find out, researchers strapped³ hatchlings into custom Lycra bathing suits, tethered to a tracking unit. They plopped each turtle into a small pool surrounded by magnetic coils. And they replicated⁴ the magnetic fields of Puerto Rico and the Cape⁵ Verde Islands, two points along the turtles' migration⁶, with equal latitudes⁷ but different longitudes⁸. The hatchlings swam opposite directions in the two trials—both being the right ones, to stay on the migratory⁹ track. The study appears in the journal Current Biology.

The researchers say turtles may calibrate¹⁰ their migratory maps by sensing the magnetic field's intensity¹¹, and its inclination¹² to the Earth's surface—a combo that gives each point of the migration a unique signature. As for any human navigators out there, you'll just have to shell out for a GPS.