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
The direction in which ocean currents flow is determined by the magnetic field of the Earth.