Compasses point north. Until one day, they’ll all point south—as they have before.

In 1905, a French geologist discovered a lava field with some very strange properties. Iron within it was magnetized in reverse. What should have been north was south.

After much pondering, he realized this could only be possible if Earth’s magnetic poles were reversed at the time the lava was flowing. When it solidified, it preserved that reversed magnetic orientation.

It took 50 years for other scientists to accept the idea that Earth could change its polarity. But when they did, a frenzy of research began to try to document the phenomenon.

First, they looked for more places where a reversed magnetic field was memorialized in iron compounds.

They found them along spreading ridges in the Atlantic, where tectonic plates pull apart and new iron-rich rock is pushed up, producing new crust for the planet.

In 1959, they mapped alternating stripes of reversed magnetic orientation in several places along the Atlantic ridge, then dated each stripe of rock.

To their surprise, they discovered that Earth had reversed polarity hundreds of times, with episodes lasting tens of thousands to millions of years before reversing again.

They also realized that Earth’s magnetic poles are always on the move, sometimes without making a full reversal.

We’ll talk more about the mess this could make on another EarthDate.