Sea turtle hatchlings leaving their hatching grounds on Florida beaches reach ocean currents by swimming to the northeast, as defined by the north of the Earth' s magnetic field  Florida hatchlings placed in a large indoor tank also swim toward the northeast
But when the tank is surrounded by an artificial magnetic field twice as strong as the Earth' s field and opposite in direction, the hatchlings swim in the direction opposite to that in which they swim without the artificial magnetic field
If a sea turtle hatches on the coast of Africa, it will swim toward the southwest.