If you could travel back in time 41,000 years to the last ice age, your compass would point south instead of north. 

That's because for a period of a few hundred years, the Earth's magnetic field was reversed. 

These reversals have happened repeatedly over the planet's history, sometimes lasting hundreds of thousands of years. 

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Regions on top of the Earth's core could behave like giant lava lamps, with blobs of rock periodically rising and falling deep inside our planet. This could affect its magnetic field and cause it to flip

We know this from the way it affects the formation of magnetic minerals, that we can now study on the Earth's surface.

Several ideas exist to explain why magnetic field reversals happen. 

One of these just became more plausible. 

My colleagues and I discovered that regions on top of the Earth's core could behave like giant lava lamps, with blobs of rock periodically rising and falling deep inside our planet.