There’s a hole in the bottom of the sea
Geologists are able to reconstruct the motion of plates in the ocean using what we call “magnetic anomalies”. Oceanic crust forms as molten rock that cools and solidifies when it touches ocean water. As it cools, it forms some minerals that respond to Earth’s magnetic field as natural magnets. These minerals record the magnetic direction at the time they formed – they become small magnets pointing to the current North and South poles.