For the last 170 years, a mysteriously weak patch of Earth’s magnetic field has grown in size, causing some geologists to think that the planet is gearing up to flip its magnetic poles. Now, buildings that were ritually burned down in Africa more than a thousand years ago are adding vital new clues to the case.

Clay fragments baked in the fires contain minerals that preserve the orientation of Earth’s magnetic field during the Iron Age, pushing back our records of these changes and offering some much-needed data from the Southern Hemisphere.

The discovery, described recently in the journal Geophysical Research Letters, also offers support for a theory about what causes the poles to flip—linking the weird weak spot in